PCI: Introduce INTx check & mask API

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 4 lines · Alexey Kardashevskiy

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 26 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Quoted 39 lines

Quoted 4 lines

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 25 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 7 lines

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Yep, that's what I'd suggest as well, add a blacklist to pci_intx_mask_supported() so this device returns false and we require an exclusive interrupt for it. Thanks,

Yep, that's what I'd suggest as well, add a blacklist to pci_intx_mask_supported() so this device returns false and we require an exclusive interrupt for it. Thanks,

Quoted 4 lines

Yep, that's what I'd suggest as well, add a blacklist to pci_intx_mask_supported() so this device returns false and we require an exclusive interrupt for it. Thanks,

BTW, we should consider supporting an MSI-only option for guests as well:

LSIs are a problem for virtualization, especially when we start having things like expander racks with slots behind bridges etc, and in some case it's better to support an MSI only setup rather than not support the virtualizing the devices at all (or at least in different partitions).

However, to do that, we either need to ensure the device can't be coerced by SW to still assert the LSI and cause trouble. This can be dealt with two ways I have in mind:

- If we don't use any of those 4 interrupts lines at all (ie, we use no LSI on the host bridge and they aren't shared with another bridge etc...), we can just mask them out in the main PIC. On Power there's no sharing between interrupt sources from different host bridges so that would work for us

- If the intermediary P2P bridge has a feature to block incoming LSIs from children (I've heard that exists, is that standard ? I haven't looked in the latest specs)

There's a third one:

- If you trust the device own mask bit

... But this is fishy since many devices -will- have some kind of backdoor via MMIO to bypass (or alter) the config space setting. In some cases the driver can even completely replace the firmware inside the device and do pretty much whatever it wants :-)

The main thing is how do we "represent" both in term of interface to qemu and qemu -> kernel interface wanting such a "no LSI" setup... not sure about that one.

Wouldn't the "no LSI" setting be a flag to the vfio configuration? So when you set up the device group, you say "this group can only do MSI". That way the interface would be sysfs and QEMU wouldn't have anything to do with it :)

Quoted 4 lines · Alexey Kardashevskiy

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 25 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Quoted 10 lines · Jan Kiszka

Quoted 4 lines · Alexey Kardashevskiy

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Is not it just a quite small optimization to not to disable interrupts on all devices which share the same IRQ but just on those who fired an interrupt? If so, do PCI devices really often share IRQs? Does not supporting this mean real slowdown on such devices?

As far as I understand, everyone who cares about performance uses MSI/MSIX, no?

Quoted 32 lines

Quoted 27 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 7 lines

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

It is a good moment to start :)
Not sure where - in VFIO or along with that PCI INTx API.

Here is that broken device:

aik@vpl2:~$ lspci -s 1:1:0.0
0001:01:00.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
aik@vpl2:~$ lspci -ns 1:1:0.0
0001:01:00.0 0200: 1425:0030

Quoted 13 lines · Alexey Kardashevskiy

Quoted 10 lines · Jan Kiszka

Quoted 4 lines · Alexey Kardashevskiy

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 5 lines

Is not it just a quite small optimization to not to disable interrupts on all devices which share the same IRQ but just on those who fired an interrupt? If so, do PCI devices really often share IRQs? Does not supporting this mean real slowdown on such devices?

As far as I understand, everyone who cares about performance uses MSI/MSIX, no?

Not everyone is blessed with MSI-only PCI devices. From my notebook:

# cat /proc/interrupts
[...]
 22: [...] IO-APIC-fasteoi   ehci_hcd:usb1, ehci_hcd:usb2

So, if I want to assign one EHCI controller to a guest, I have to disable the other as well. The same can happen quickly if you attach a few legacy PCI adapters to a system and want to pass them through.

Quoted 38 lines

Quoted 30 lines

Quoted 25 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 7 lines

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Quoted 2 lines

It is a good moment to start :)
Not sure where - in VFIO or along with that PCI INTx API.

At PCI level as the API is VFIO agnostic (it was introduced for "classic" KVM device assignment, in fact).

Quoted 5 lines

Here is that broken device:

aik@vpl2:~$ lspci -s 1:1:0.0
0001:01:00.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
aik@vpl2:~$ lspci -ns 1:1:0.0
0001:01:00.0 0200: 1425:0030

A patch to add the infrastructure as well would be even more welcome. :) You could have a look at drivers/pci/quirks.c for patterns how to do this.

Quoted 26 lines · Jan Kiszka

Quoted 13 lines · Alexey Kardashevskiy

Quoted 10 lines · Jan Kiszka

Quoted 4 lines · Alexey Kardashevskiy

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 5 lines

Is not it just a quite small optimization to not to disable interrupts on all devices which share the same IRQ but just on those who fired an interrupt? If so, do PCI devices really often share IRQs? Does not supporting this mean real slowdown on such devices?

As far as I understand, everyone who cares about performance uses MSI/MSIX, no?

Not everyone is blessed with MSI-only PCI devices. From my notebook:

# cat /proc/interrupts
[...]
 22: [...] IO-APIC-fasteoi   ehci_hcd:usb1, ehci_hcd:usb2

Quoted 3 lines

So, if I want to assign one EHCI controller to a guest, I have to disable the other as well. The same can happen quickly if you attach a few legacy PCI adapters to a system and want to pass them through.

Why? vfio-pci receives interrupt, disables it, handles it, enables interrupt back. Yes, handling is a bit longer and includes passing interrupt to QEMU and then to the guest (can be optimized to avoid QEMU) and waiting for EOI notification but this is all the difference.

Does the current kernel use INTx bit for your USB controllers now, without any KVM, etc?

So, is it just an optimization or it is something bigger that I missed?

Quoted 51 lines

Quoted 40 lines

Quoted 32 lines

Quoted 27 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 7 lines

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Quoted 2 lines

It is a good moment to start :)
Not sure where - in VFIO or along with that PCI INTx API.

At PCI level as the API is VFIO agnostic (it was introduced for "classic" KVM device assignment, in fact).

Quoted 5 lines

Here is that broken device:

aik@vpl2:~$ lspci -s 1:1:0.0
0001:01:00.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
aik@vpl2:~$ lspci -ns 1:1:0.0
0001:01:00.0 0200: 1425:0030

Quoted 2 lines

A patch to add the infrastructure as well would be even more welcome. :) You could have a look at drivers/pci/quirks.c for patterns how to do this.

I am not sure yet that we need this feature at all ;) I would rather prefer to have some way to disable it in VFIO rather than to add yet another quirk for the feature which nobody uses at the moment. Really, this device supports MSI/MSIX and in real life nobody is going to use INTx on it. The only need for it is testing.

Quoted 31 lines · Jan Kiszka

Quoted 29 lines · Alexey Kardashevskiy

Quoted 13 lines · Jan Kiszka

Quoted 10 lines · Alexey Kardashevskiy

Quoted 4 lines

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

Quoted 5 lines

Is not it just a quite small optimization to not to disable interrupts on all devices which share the same IRQ but just on those who fired an interrupt? If so, do PCI devices really often share IRQs? Does not supporting this mean real slowdown on such devices?

As far as I understand, everyone who cares about performance uses MSI/MSIX, no?

Not everyone is blessed with MSI-only PCI devices. From my notebook:

# cat /proc/interrupts
[...]
 22: [...] IO-APIC-fasteoi   ehci_hcd:usb1, ehci_hcd:usb2

So, if I want to assign one EHCI controller to a guest, I have to disable the other as well. The same can happen quickly if you attach a few legacy PCI adapters to a system and want to pass them through.

Quoted 3 lines

Why? vfio-pci receives interrupt, disables it, handles it, enables interrupt back. Yes, handling is a bit longer and includes passing interrupt to QEMU and then to the guest (can be optimized to avoid QEMU) and waiting for EOI notification but this is all the difference.

You can disable the complete IRQ line as you cannot predict when the untrusted device driver that VFIO, KVM, or UIO serves will finally decide to silence the IRQ reason in hardware. If you did this, you risk a DoS attack on those other devices.

Quoted 1 line

Does the current kernel use INTx bit for your USB controllers now, without any KVM, etc?

No, it is only used for KVM device assignment when it grabs a device and uio_pci_generic. If a host driver uses the device, and can silence interrupts in a device-specific way.

Quoted 2 lines

So, is it just an optimization or it is something bigger that I missed?

It is not an optimization but an essential feature to support INTx sharing between an untrusted device driver and some other driver.

Quoted 49 lines

Quoted 49 lines

Quoted 38 lines

Quoted 30 lines

Quoted 25 lines

And there is some API to support that (commit a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter BUT the adapter does not set PCI_STATUS_INTERRUPT so pci_check_and_set_intx_mask() never changes PCI_COMMAND and INTx does not work on it when we use it as VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an interrupt handler and Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3 This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command register is a 0 and this Interrupt Status bit is a 1, will the device’s/function’s INTx# signal be asserted. Setting the Interrupt
Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Quoted 4 lines

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in VFIO-PCI somehow.

Quoted 7 lines

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy, or if our detection algorithm is unreliable. It basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.

Quoted 2 lines

It is a good moment to start :)
Not sure where - in VFIO or along with that PCI INTx API.

At PCI level as the API is VFIO agnostic (it was introduced for "classic" KVM device assignment, in fact).

Quoted 5 lines

Here is that broken device:

aik@vpl2:~$ lspci -s 1:1:0.0
0001:01:00.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single Port Adapter
aik@vpl2:~$ lspci -ns 1:1:0.0
0001:01:00.0 0200: 1425:0030

Quoted 7 lines

Quoted 2 lines

A patch to add the infrastructure as well would be even more welcome. :) You could have a look at drivers/pci/quirks.c for patterns how to do this.

I am not sure yet that we need this feature at all ;) I would rather prefer to have some way to disable it in VFIO rather than to add yet another quirk for the feature which nobody uses at the moment. Really, this device supports MSI/MSIX and in real life nobody is going to use INTx on it. The only need for it is testing.

These are wrong assumptions, both that it has to be addressed at VFIO level and that it has no serious use case. We will need this feature for quite a while until legacy PCI finally died. Bets are taken when this will happen, but I would be careful with any date in this decade. ;)

25.05.2012 20:43, Jan Kiszka написал:

Quoted 32 lines · Alexey Kardashevskiy

Quoted 31 lines · Jan Kiszka

Quoted 29 lines · Alexey Kardashevskiy

Quoted 13 lines · Jan Kiszka

Quoted 10 lines · Alexey Kardashevskiy

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[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and PCI_STATUS registers.

Yes, 2.3 introduced this. Masking is done via command register, checking if the source was the PCI in question via the status register. The latter is important for supporting IRQ sharing - and that's why we introduced this masking API to the PCI layer.

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Is not it just a quite small optimization to not to disable interrupts on all devices which share the same IRQ but just on those who fired an interrupt? If so, do PCI devices really often share IRQs? Does not supporting this mean real slowdown on such devices?

As far as I understand, everyone who cares about performance uses MSI/MSIX, no?

Not everyone is blessed with MSI-only PCI devices. From my notebook:

# cat /proc/interrupts
[...]
 22: [...] IO-APIC-fasteoi   ehci_hcd:usb1, ehci_hcd:usb2

So, if I want to assign one EHCI controller to a guest, I have to disable the other as well. The same can happen quickly if you attach a few legacy PCI adapters to a system and want to pass them through.

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Why? vfio-pci receives interrupt, disables it, handles it, enables interrupt back. Yes, handling is a bit longer and includes passing interrupt to QEMU and then to the guest (can be optimized to avoid QEMU) and waiting for EOI notification but this is all the difference.

You can disable the complete IRQ line as you cannot predict when the untrusted device driver that VFIO, KVM, or UIO serves will finally decide to silence the IRQ reason in hardware. If you did this, you risk a DoS attack on those other devices.

Untrusted device still can pull down (or up? do not remember :) ) hardware INT# line, stop other devices on this line and you cannot do anything about it. How does INTx help if the device is that broken?

Since PCI 2.3, this bit is mandatory, and it should be independent of the masking bit. The question is, if your device is supposed to support 2.3, thus is just buggy,

or if our detection algorithm is unreliable. It
basically builds on the assumption that, if we can flip the mask bit, the feature should be present. I guess that is the best we can do. Maybe we can augment this with a blacklist of devices that "support" flipping without actually providing the feature.