Prev: [PATCH] max730x: make pullups configurable via platformdata
Next: Keep SMP VMs more in sync on unstable TSC
From: Avi Kivity on 15 Jun 2010 04:10
On 06/15/2010 10:34 AM, Zachary Amsden wrote:
> When CPUs with unstable TSCs enter deep C-state, TSC may stop
> running. This causes us to require resynchronization. Since
> we can't tell when this may potentially happen, we assume the
> worst by forcing re-compensation for it at every point the VCPU
> task is descheduled.
>
> Signed-off-by: Zachary Amsden<zamsden(a)redhat.com>
> ---
> arch/x86/kvm/x86.c | 11 +++++++++++
> 1 files changed, 11 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c8289d0..618c435 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> kvm_x86_ops->vcpu_put(vcpu);
> kvm_put_guest_fpu(vcpu);
> +
> vcpu->arch.last_host_tsc = native_read_tsc();
> +
> + /*
> + * When potentially leaving a CPU with unstable TSCs, we risk
> + * that the CPU enters deep C-state. If it does, the TSC may
> + * go out of sync but we will not recalibrate because the test
> + * vcpu->cpu != cpu can not detect this condition. So set
> + * vcpu->cpu = -1 to force the recalibration above.
> + */
> + if (check_tsc_unstable())
> + vcpu->cpu = -1;
> }
>

That will cause us to miss a vmclear later on. Also it invalidates an
invariant that the per-cpu list vcpus_on_cpu has all the vcpus with
vcpu->cpu == cpu on this cpu (try saything that fast).

--
error compiling committee.c: too many arguments to function

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Zachary Amsden on 15 Jun 2010 04:20
On 06/14/2010 10:09 PM, Avi Kivity wrote:
> On 06/15/2010 10:34 AM, Zachary Amsden wrote:
>> When CPUs with unstable TSCs enter deep C-state, TSC may stop
>> running. This causes us to require resynchronization. Since
>> we can't tell when this may potentially happen, we assume the
>> worst by forcing re-compensation for it at every point the VCPU
>> task is descheduled.
>>
>> Signed-off-by: Zachary Amsden<zamsden(a)redhat.com>
>> ---
>> arch/x86/kvm/x86.c | 11 +++++++++++
>> 1 files changed, 11 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c8289d0..618c435 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>> {
>> kvm_x86_ops->vcpu_put(vcpu);
>> kvm_put_guest_fpu(vcpu);
>> +
>> vcpu->arch.last_host_tsc = native_read_tsc();
>> +
>> + /*
>> + * When potentially leaving a CPU with unstable TSCs, we risk
>> + * that the CPU enters deep C-state. If it does, the TSC may
>> + * go out of sync but we will not recalibrate because the test
>> + * vcpu->cpu != cpu can not detect this condition. So set
>> + * vcpu->cpu = -1 to force the recalibration above.
>> + */
>> + if (check_tsc_unstable())
>> + vcpu->cpu = -1;
>> }
>
> That will cause us to miss a vmclear later on. Also it invalidates an
> invariant that the per-cpu list vcpus_on_cpu has all the vcpus with
> vcpu->cpu == cpu on this cpu (try saything that fast).
>

Blasted SVM / VMX differences! There are other, less elegant ways to
accomplish this task however.

Good catch, BTW.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Jason Wang on 16 Jun 2010 04:10
Zachary Amsden wrote:
> When CPUs with unstable TSCs enter deep C-state, TSC may stop
> running. This causes us to require resynchronization. Since
> we can't tell when this may potentially happen, we assume the
> worst by forcing re-compensation for it at every point the VCPU
> task is descheduled.
>
>
Is there any method to do the compensation on host TSC when recovering
from C-state? It should be simpler than do it on guest TSC.
> Signed-off-by: Zachary Amsden <zamsden(a)redhat.com>
> ---
> arch/x86/kvm/x86.c | 11 +++++++++++
> 1 files changed, 11 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c8289d0..618c435 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> {
> kvm_x86_ops->vcpu_put(vcpu);
> kvm_put_guest_fpu(vcpu);
> +
> vcpu->arch.last_host_tsc = native_read_tsc();
> +
> + /*
> + * When potentially leaving a CPU with unstable TSCs, we risk
> + * that the CPU enters deep C-state. If it does, the TSC may
> + * go out of sync but we will not recalibrate because the test
> + * vcpu->cpu != cpu can not detect this condition. So set
> + * vcpu->cpu = -1 to force the recalibration above.
> + */
> + if (check_tsc_unstable())
> + vcpu->cpu = -1;
> }
>
> static int is_efer_nx(void)
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Glauber Costa on 16 Jun 2010 09:30
On Mon, Jun 14, 2010 at 09:34:06PM -1000, Zachary Amsden wrote:
> When CPUs with unstable TSCs enter deep C-state, TSC may stop
> running. This causes us to require resynchronization. Since
> we can't tell when this may potentially happen, we assume the
> worst by forcing re-compensation for it at every point the VCPU
> task is descheduled.
can't we just compensate everytime check_tsc_unstable() is true?


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Zachary Amsden on 16 Jun 2010 14:50
On 06/15/2010 10:10 PM, Jason Wang wrote:
> Zachary Amsden wrote:
>
>> When CPUs with unstable TSCs enter deep C-state, TSC may stop
>> running. This causes us to require resynchronization. Since
>> we can't tell when this may potentially happen, we assume the
>> worst by forcing re-compensation for it at every point the VCPU
>> task is descheduled.
>>
>>
>>
> Is there any method to do the compensation on host TSC when recovering
> from C-state? It should be simpler than do it on guest TSC.
>

Actually, yes, we could re-calibrate from HPET. Considering this patch
is already broken, maybe that's a better idea.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
 |  Next  |  Last
Pages: 1 2
Prev: [PATCH] max730x: make pullups configurable via platformdata
Next: Keep SMP VMs more in sync on unstable TSC