x86: Make x2APIC support depend on interrupt remapping or guest support
PATCH 1/1 x86: Make x2APIC support depend on interrupt remapping or guest support
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable IRQ_REMAP without having to give up x2APIC support.
--- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
arch/x86/Kconfig +1 -1
@@ -310,7 +310,7 @@ config SMP config X86_X2APIC bool "Support x2apic" - depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP + depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST) ---help--- This enables x2apic support on CPUs that have this feature.
Il 05/07/2014 09:47, Jan Kiszka ha scritto:
Quoted 18 lines
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.Signed-off-by Jan Kiszka <[email protected]>--- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)arch/x86/Kconfig +0
-0
arch/x86/Kconfig config SMP · line 316
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arch/x86/Kconfig +1
-1@@ -310,7 +310,7 @@ config SMP config X86_X2APIC bool "Support x2apic" - depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP + depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST) ---help--- This enables x2apic support on CPUs that have this feature.
Makes sense.
Quoted 5 lines · Jan Kiszka
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.
FWIW I did similar thing back when I added x2apic to KVM:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
But was advised against it.
Quoted 13 lines
Signed-off-by Jan Kiszka <[email protected]>--- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)arch/x86/Kconfig +0
-0
arch/x86/Kconfig config SMP · line 316
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arch/x86/Kconfig +1
-1@@ -310,7 +310,7 @@ config SMP config X86_X2APIC bool "Support x2apic" - depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP + depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST) ---help--- This enables x2apic support on CPUs that have this feature.
--
Gleb.
Quoted 9 lines · Gleb Natapov
Quoted 5 lines · Jan Kiszka
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.FWIW I did similar thing back when I added x2apic to KVM:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
But was advised against it.
I don't get the point from that thread.
However, this change is just formalizing a configuration that is already being used: there is no interrupt remapping available with KVM (yet), but we do run our guests in x2APIC mode most of the time. Interrupt remapping code in kernels tailored as KVM guest is dead code, right now.
Jan
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Signed-off-by Jan Kiszka <[email protected]>--- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)arch/x86/Kconfig +0
-0arch/x86/Kconfig config SMP · line 316
Quoted 8 lines
arch/x86/Kconfig +1
-1@@ -310,7 +310,7 @@ config SMP config X86_X2APIC bool "Support x2apic" - depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP + depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST) ---help--- This enables x2apic support on CPUs that have this feature.--
Gleb.
Quoted 4 lines · Jan Kiszka
Quoted 3 lines · Gleb Natapov
Quoted 2 lines · Jan Kiszka
From: Jan Kiszka <[email protected]>
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We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.FWIW I did similar thing back when I added x2apic to KVM:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
But was advised against it.I don't get the point from that thread.
It is not architectural to use x2apic without irq remapping, so Suresh from Intel felt that it should not be allowed to create a kernel that can run on real HW and has non architectural configuration compiled in, though only when running on a VM x2apic will ever be enabled without irq remapping. I didn't argue about it to much back then. If HYPERVISOR_GUEST guaranties that resulting kernel can run only on a VM (does it?) this objection does not hold any more.
Quoted 5 lines
However, this change is just formalizing a configuration that is already being used: there is no interrupt remapping available with KVM (yet), but we do run our guests in x2APIC mode most of the time. Interrupt remapping code in kernels tailored as KVM guest is dead code, right now.
Yes. I am just saying why things are the way they are.
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Jan
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Signed-off-by Jan Kiszka <[email protected]>--- arch/x86/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)arch/x86/Kconfig +0
-0arch/x86/Kconfig config SMP · line 316
Quoted 8 lines
arch/x86/Kconfig +1
-1@@ -310,7 +310,7 @@ config SMP config X86_X2APIC bool "Support x2apic" - depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP + depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST) ---help--- This enables x2apic support on CPUs that have this feature.--
Gleb.
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Gleb.
Quoted 8 lines · Gleb Natapov
Quoted 7 lines · Jan Kiszka
Quoted 6 lines · Gleb Natapov
Quoted 5 lines · Jan Kiszka
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.
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FWIW I did similar thing back when I added x2apic to KVM:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
But was advised against it.I don't get the point from that thread.
It is not architectural to use x2apic without irq remapping, so Suresh
Slightly off-topic for this patch: Is this dependency documented somewhere? I guess there are broken systems where it can fail, but none of the (Intel) boxes I tried so far had problems with x2APIC mode on (given APIC IDs < 255) and IRQ remapping disabled.
Quoted 6 lines
from Intel felt that it should not be allowed to create a kernel that can run on real HW and has non architectural configuration compiled in, though only when running on a VM x2apic will ever be enabled without irq remapping. I didn't argue about it to much back then. If
HYPERVISOR_GUESTguaranties that resulting kernel can run only on a VM (does it?) this objection does not hold any more.
Yes, indeed. Your patch suggested to remove the dependency, this one preserves it. There will still be no real hardware able to turn on x2APIC without IRQ remapping enabled as well (though I'm hoping to overcome this for Jailhouse scenarios until we can hand over DMAR units from Linux to the hypervisor).
Jan
Quoted 13 lines · Jan Kiszka
Quoted 12 lines · Gleb Natapov
Quoted 11 lines · Jan Kiszka
Quoted 9 lines · Gleb Natapov
Quoted 5 lines · Jan Kiszka
From: Jan Kiszka <[email protected]>
We are able to use x2APIC mode in the absence of interrupt remapping on certain hypervisors. So it if fine to disable
IRQ_REMAPwithout having to give up x2APIC support.FWIW I did similar thing back when I added x2apic to KVM:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
But was advised against it.
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I don't get the point from that thread.
It is not architectural to use x2apic without irq remapping, so Suresh
Slightly off-topic for this patch: Is this dependency documented somewhere? I guess there are broken systems where it can fail, but none of the (Intel) boxes I tried so far had problems with x2APIC mode on (given APIC IDs < 255) and IRQ remapping disabled.
Not sure I saw that written, but I was told many times that this is so.
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Gleb.