KVM & VMX root mode
Hi,
We have had several reports of users saying our product VirtualBox crashes their (Linux) host. It turned out that KVM is responsible, because it leaves the CPU in VMX root mode.
Doing this changes the way the cpu operates: updates to reserved bits in control registers cause general protection faults.
We hit such cases on two occasions; clearing the X86_CR4_VMXE bit in CR4 and turning off paging.
I have added a workaround to detect VMX root mode, simply refuse to run and recommend that the user disables KVM.
I would however appreciate if you could fix KVM instead.
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Quoted 6 lines · Sander van Leeuwen
Hi,
We have had several reports of users saying our product VirtualBox crashes their (Linux) host. It turned out that KVM is responsible, because it leaves the CPU in VMX root mode.
Quoted 10 lines
Doing this changes the way the cpu operates: updates to reserved bits in control registers cause general protection faults.
We hit such cases on two occasions; clearing theX86_CR4_VMXEbit in CR4 and turning off paging.I have added a workaround to detect VMX root mode, simply refuse to run and recommend that the user disables KVM.
I would however appreciate if you could fix KVM instead.
Simply '/sbin/rmmod kvm-intel' and kvm will exit vmx root mode.
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Quoted 1 line
Simply '
/sbin/rmmodkvm-intel' and kvm will exit vmx root mode.
Yes, but this is very poor solution. This basically disables KVM. It doesn't enables users to run VirtualBox and KVM side-by-side, at the same time.
I would like to see a really fixed solution.
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Quoted 8 lines · Alexey Eremenko
Quoted 2 lines
Simply '
/sbin/rmmodkvm-intel' and kvm will exit vmx root mode.Yes, but this is very poor solution. This basically disables KVM. It doesn't enables users to run VirtualBox and KVM side-by-side, at the same time.
If VirtualBox uses vmx, then it can't be run side-by-side with kvm.
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Quoted 2 lines
If VirtualBox uses vmx, then it can't be run side-by-side with kvm.
VirtualBox doesn't uses VMX by default, but can take advantage of.
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Hi Avi,
Our non-vmx mode fails, because the cpu is in vmx root mode.
Two products that use vt-x for virtualization could perfectly co-exist if both comply with the way Intel recommends people to use vt-x. See figure 19.1 in chapter 19.4 of the 'Intel 64 and IA-32 Architectures Software Developer's Manual'. VirtualBox is programmed to follow these rules and therefor allows any other virtualizer to run side-by-side.
Currently KVM prevents us from using our generic virtualization engine and does not allow anybody else to use the vt-x extensions (without explicitely leaving vmx root mode).
As your product is included in the mainline Linux kernel and enabled by default, it would be nice if you could follow Intel's recommendations.
Thank you.
Quoted 22 lines · Avi Kivity
Quoted 7 lines · Sander van Leeuwen
Hi,
We have had several reports of users saying our product VirtualBox crashes their (Linux) host. It turned out that KVM is responsible, because
it leaves the CPU in VMX root mode.Quoted 10 lines
Doing this changes the way the cpu operates: updates to reserved bits in control registers cause general protection faults.
We hit such cases on two occasions; clearing theX86_CR4_VMXEbit in CR4 and turning off paging.I have added a workaround to detect VMX root mode, simply refuse to run and recommend that the user disables KVM.
I would however appreciate if you could fix KVM instead.Simply '
/sbin/rmmodkvm-intel' and kvm will exit vmx root mode.If VirtualBox uses vmx, then it can't be run side-by-side with kvm.
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Quoted 10 lines · Sander van Leeuwen
Hi Avi,
Our non-vmx mode fails, because the cpu is in vmx root mode.
Two products that use vt-x for virtualization could perfectly co-exist if both comply with the way Intel recommends people to use vt-x. See figure 19.1 in chapter 19.4 of the 'Intel 64 and IA-32 Architectures Software Developer's Manual'. VirtualBox is programmed to follow these rules and therefor allows any other virtualizer to run side-by-side.
As far as I understand, kvm follows these rules. It enables vmx when loaded and disables then when unloaded.
Quoted 4 lines
Currently KVM prevents us from using our generic virtualization engine and does not allow anybody else to use the vt-x extensions (without explicitely leaving vmx root mode).
Well, obviously kvm can't operate if you disable cr4.vmxe and/or switch paging off. The two solutions are not run-time compatible. I don't see why this is a problem as you should simply not run the product you aren't using, and everything should just work.
Quoted 3 lines
As your product is included in the mainline Linux kernel and enabled by default, it would be nice if you could follow Intel's recommendations.
kvm isn't enabled by default. It requires explicit user action to enter vmx mode ('modprobe kvm-intel').
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Avi,
I'd like to make clear that we are talking about the situation where kvm is *not* being used by any application.
It's my understanding that in some (many?) distributions kvm is actually active on a permanent basis. I can't say whether this was done by default or explicitly by the user. There is however no need to activate vmx root mode when there is no client program that is using the kvm extension.
From your comments I understand that you enable vmx root mode when the
kvm module is loaded. Regardless of whether an application actually wishes to use the kvm extension.
Quoted 36 lines · Avi Kivity
Quoted 11 lines · Sander van Leeuwen
Hi Avi,
Our non-vmx mode fails, because the cpu is in vmx root mode.
Two products that use vt-x for virtualization could perfectly co-exist if both comply with the way Intel recommends people to use vt-x.
See figure 19.1 in chapter 19.4 of the 'Intel 64 and IA-32 Architectures Software Developer's Manual'. VirtualBox is programmed to follow these rules and therefor allows any other virtualizer to run side-by-side.As far as I understand, kvm follows these rules. It enables vmx when loaded and disables then when unloaded.
Quoted 5 lines
Currently KVM prevents us from using our generic virtualization engine and does not allow anybody else to use the vt-x extensions (without
explicitely leaving vmx root mode).Well, obviously kvm can't operate if you disable cr4.vmxe and/or switch paging off. The two solutions are not run-time compatible. I don't see why this is a problem as you should simply not run the product you aren't using, and everything should just work.
Quoted 4 lines
As your product is included in the mainline Linux kernel and enabled by default, it would be nice if you could follow Intel's recommendations.
kvm isn't enabled by default. It requires explicit user action to enter vmx mode ('modprobe kvm-intel').
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Quoted 1 line · Sander
I'd like to make clear that we are talking about the situation where kvm
is *not* being used by any application.
So, basically I can't use different virtualizers at the same time. Too bad...
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Quoted 11 lines · Sander van Leeuwen
Avi,
I'd like to make clear that we are talking about the situation where kvm is *not* being used by any application.
It's my understanding that in some (many?) distributions kvm is actually active on a permanent basis. I can't say whether this was done by default or explicitly by the user. There is however no need to activate vmx root mode when there is no client program that is using the kvm extension.
Quoted 3 lines
From your comments I understand that you enable vmx root mode when the kvm module is loaded. Regardless of whether an application actually wishes to use the kvm extension.
Yes. The assumption is that if kvm is loaded, then there is an intent to use it. This is similar to an ethernet device being claimed by the driver that supports it, even if no application actually uses that network interface.
We could change kvm to only claim vmx on first use, but that would reduce reliability.
My position is that:
- if you with to run something that conflicts with kvm enabling vmx, you should unload kvm, or not load it in the first place. This is not a difficult operation.
- if you have an issue with distributions enabling kvm by default, then talk to these distributions. kvm itself does not enable vmx by default.
Quoted 43 lines
Quoted 36 lines · Avi Kivity
Quoted 11 lines · Sander van Leeuwen
Hi Avi,
Our non-vmx mode fails, because the cpu is in vmx root mode.
Two products that use vt-x for virtualization could perfectly co-exist if both comply with the way Intel recommends people to use vt-x.
See figure 19.1 in chapter 19.4 of the 'Intel 64 and IA-32 Architectures Software Developer's Manual'. VirtualBox is programmed to follow these rules and therefor allows any other virtualizer to run side-by-side.As far as I understand, kvm follows these rules. It enables vmx when loaded and disables then when unloaded.
Quoted 5 lines
Currently KVM prevents us from using our generic virtualization engine and does not allow anybody else to use the vt-x extensions (without
explicitely leaving vmx root mode).Well, obviously kvm can't operate if you disable cr4.vmxe and/or switch paging off. The two solutions are not run-time compatible. I don't see why this is a problem as you should simply not run the product you aren't using, and everything should just work.
Quoted 4 lines
As your product is included in the mainline Linux kernel and enabled by default, it would be nice if you could follow Intel's recommendations.
kvm isn't enabled by default. It requires explicit user action to enter vmx mode ('modprobe kvm-intel').
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Quoted 24 lines · Avi Kivity
Quoted 11 lines · Sander van Leeuwen
Avi,
I'd like to make clear that we are talking about the situation where kvm is *not* being used by any application.
It's my understanding that in some (many?) distributions kvm is actually active on a permanent basis. I can't say whether this was done by default or explicitly by the user. There is however no need to activate vmx root mode when there is no client program that is using the kvm extension.
Quoted 3 lines
From your comments I understand that you enable vmx root mode when the kvm module is loaded. Regardless of whether an application actually wishes to use the kvm extension.
Yes. The assumption is that if kvm is loaded, then there is an intent to use it. This is similar to an ethernet device being claimed by the driver that supports it, even if no application actually uses that network interface.
We could change kvm to only claim vmx on first use, but that would reduce reliability.
Quoted 6 lines
My position is that:
- if you with to run something that conflicts with kvm enabling vmx, you should unload kvm, or not load it in the first place. This is not a difficult operation.
- if you have an issue with distributions enabling kvm by default, then talk to these distributions. kvm itself does not enable vmx by default.
So with my distributor / maintainer hat on...
In Fedora we enable KVM kernel modules by default because upstream LKML has decided that KVM is the standard for exposing hardware virt capabilities from the kernel to userspace. Asking the user to choose which kernel modules they load for virt is not really at all pratical because most users do not have the neccessary knowledge to be able to make an informed decision on this question. For this reason, in Fedora our general policy is to evaluate the options provided in the /upstream/ kernel, identify the best driver for any particular hardware capability & enable it by default. Thus KVM kernel mods are enabled by default if you install the KVM userspace.
While one could in theory change KVM to only activate VMX when a guest is started this wouldn't give a significant improvement to the user experiance. Instead of being able to rely on KVM working, it now may or may not work depending on whether you happen to have a VirtualBox guest running or not & vica-verca. You still would not be able to run guests from both concurrently which should surely be the real goal. The only way I see the latter being achieved is if VirtualBox were to leverage the existing KVM kernel APIs to access the underlying hardware virt capabilities.
Regards,
Dan.
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Quoted 9 lines · Daniel P. Berrange
While one could in theory change KVM to only activate VMX when a guest is started this wouldn't give a significant improvement to the user experiance. Instead of being able to rely on KVM working, it now may or may not work depending on whether you happen to have a VirtualBox guest running or not & vica-verca. You still would not be able to run guests from both concurrently which should surely be the real goal. The only way I see the latter being achieved is if VirtualBox were to leverage the existing KVM kernel APIs to access the underlying hardware virt capabilities.
You are mistaken. VT-x allows multiple clients to work concurrently. There's only one catch: all those
clients should follow certain guidelines.
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Quoted 16 lines · Sander van Leeuwen
Quoted 6 lines · Daniel P. Berrange
While one could in theory change KVM to only activate VMX when a guest is
started this wouldn't give a significant improvement to the user experiance.
Instead of being able to rely on KVM working, it now may or may not work depending on whether you happen to have a VirtualBox guestQuoted 9 lines
running or not &
vica-verca. You still would not be able to run guests from both concurrently
which should surely be the real goal. The only way I see the latter being
achieved is if VirtualBox were to leverage the existing KVM kernel APIs to
access the underlying hardware virt capabilities.
Quoted 3 lines
You are mistaken. VT-x allows multiple clients to work concurrently. There's only one catch: all those
clients should follow certain guidelines.
Currently, kvm assumes that it is the only user that issues vmptrld instructions. There may me other such assumptions in the code.
There is also the question of who issues the vmxon/vmxoff instructions and set the magic msr bits that enable kvm. You can't have two hypervisors doing that; that would be racy. It should be possible to abstract those bits out to something outside kvm, but I don't see that happening.
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Quoted 7 lines · Avi Kivity
Quoted 4 lines · Sander van Leeuwen
You are mistaken. VT-x allows multiple clients to work concurrently. There's only one catch: all those
clients should follow certain guidelines.
Quoted 10 lines
Currently, kvm assumes that it is the only user that issues vmptrld instructions. There may me other such assumptions in the code.
There is also the question of who issues the vmxon/vmxoff instructions and set the magic msr bits that enable kvm. You can't have two hypervisors doing that; that would be racy. It should be possible to abstract those bits out to something outside kvm, but I don't see that happening.
VirtualBox currently always executes vmxon when it wants to execute some guest code. When leaving ring 0 it issues the corresponding vmxoff. While that may increase world switch overhead, it has not turned out to be an issue for us. I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
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Quoted 18 lines · Sander van Leeuwen
Quoted 7 lines · Avi Kivity
Quoted 4 lines · Sander van Leeuwen
You are mistaken. VT-x allows multiple clients to work concurrently. There's only one catch: all those
clients should follow certain guidelines.Quoted 10 lines
Currently, kvm assumes that it is the only user that issues vmptrld instructions. There may me other such assumptions in the code.
There is also the question of who issues the vmxon/vmxoff instructions and set the magic msr bits that enable kvm. You can't have two hypervisors doing that; that would be racy. It should be possible to abstract those bits out to something outside kvm, but I don't see that happening.
Quoted 5 lines
VirtualBox currently always executes vmxon when it wants to execute some guest code. When leaving
ring 0 it issues the corresponding vmxoff. While that may increase world switch overhead, it has not turned
out to be an issue for us.
The overhead will increase with newer hardware, as more state is cached on-chip. It's probably okay when deploying to a desktop but i don't think it is good enough for a server.
Quoted 3 lines
I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
I think the easiest way forward is for you to unload kvm-intel (or kvm-amd) when you load, and to reload it when you unload.
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Quoted 6 lines · Avi Kivity
Quoted 3 lines
I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
I think the easiest way forward is for you to unload kvm-intel (or kvm-amd) when you load, and to reload it when you unload.
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way
as it breaks VirtualBox.
We have tried this ourselves and others have reported the same:
- http://www.vmware.com/community/message.jspa?messageID=718184
- http://www.redhat.com/archives/rhl-list/2007-June/msg02501.html
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Quoted 7 lines · Sander van Leeuwen
Quoted 6 lines · Avi Kivity
Quoted 3 lines
I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
I think the easiest way forward is for you to unload kvm-intel (or kvm-amd) when you load, and to reload it when you unload.
Quoted 8 lines
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way
as it breaks VirtualBox.We have tried this ourselves and others have reported the same:
- http://www.vmware.com/community/message.jspa?messageID=718184
- http://www.redhat.com/archives/rhl-list/2007-June/msg02501.html
This is unsurprising. But I believe the same solution (rmmod kvm-intel) will work for VMware as well as for VirtualBox, or anything else that wants to use VT.
Can you explain what is wrong with it? It is simple, requires no kernel changes, and has no present and potential future performance impact. It will work on all kernel versions, unlike any change you may wish us to make.
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Quoted 10 lines · Avi Kivity
Quoted 8 lines · Sander van Leeuwen
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way
as it breaks VirtualBox.We have tried this ourselves and others have reported the same:
- http://www.vmware.com/community/message.jspa?messageID=718184
- http://www.redhat.com/archives/rhl-list/2007-June/msg02501.html
Quoted 9 lines
This is unsurprising. But I believe the same solution (rmmod kvm-intel) will work for VMware as well as for VirtualBox, or anything else that wants to use VT.
Can you explain what is wrong with it? It is simple, requires no kernel changes, and has no present and potential future performance impact. It will work on all kernel versions, unlike any change you may wish us to make.
This was just an added note. More meant for distribution maintainers who feel it is necessary to activate KVM by default.
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Quoted 17 lines · Sander van Leeuwen
Quoted 15 lines · Avi Kivity
Quoted 9 lines · Sander van Leeuwen
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way
as it breaks VirtualBox.We have tried this ourselves and others have reported the same:
- http://www.vmware.com/community/message.jspa?messageID=718184
- http://www.redhat.com/archives/rhl-list/2007-June/msg02501.htmlThis is unsurprising. But I believe the same solution (rmmod kvm-intel) will work for VMware as well as for VirtualBox, or anything else that wants to use VT.
Quoted 9 lines
Quoted 6 lines
Can you explain what is wrong with it? It is simple, requires no kernel changes, and has no present and potential future performance impact. It will work on all kernel versions, unlike any change you may wish us to make.
This was just an added note. More meant for distribution maintainers who feel it is necessary to activate KVM by default.
Well, you still haven't explained what is wrong with my proposal. Can we make some progress instead of walking in circles?
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Quoted 10 lines · Sander van Leeuwen
Quoted 6 lines · Avi Kivity
Quoted 3 lines
I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
I think the easiest way forward is for you to unload kvm-intel (or kvm-amd) when you load, and to reload it when you unload.
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way
as it breaks VirtualBox.
Can VirtualBox and VMware server 1.3 on 64 bits run at the same time?
Regards,
Anthony Liguori
Quoted 5 lines
We have tried this ourselves and others have reported the same:
- http://www.vmware.com/community/message.jspa?messageID=718184
- http://www.redhat.com/archives/rhl-list/2007-June/msg02501.html
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Quoted 8 lines · Anthony Liguori
Quoted 6 lines · Sander van Leeuwen
Quoted 4 lines · Avi Kivity
Quoted 4 lines
I think it's a bit risky to use somebody else's vmxon pointer.
You have a point with the msr updates though.
Quoted 10 lines
Quoted 6 lines
Quoted 3 lines
I think the easiest way forward is for you to unload kvm-intel (or kvm-amd) when you load, and to reload it when you unload.
You may want to reconsider as KVM breaks VMWare server 1.3 on 64 bits linux hosts as well. In exactly the same way as it breaks VirtualBox.
Can VirtualBox and VMware server 1.3 on 64 bits run at the same time?
In theory they can, although I haven't tried myself. We've heard from people that they managed to run the 32 bits version side by side.
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Quoted 4 lines · Sander van Leeuwen
Anthony Liguori wrote:
In theory they can, although I haven't tried myself. We've heard from people that they managed to run
the 32 bits version side by side.
Only if VMware disables VMX root mode. 32 bit VMware doesn't use VT/SVM by default IIUC.
Regards,
Anthony Liguori
Quoted 9 lines
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