Re: KVM: SVM: Disable Lazy FPU optimization because of regressions

[Mike Lampard <mike-pBbpZId3k0Zx3z9c7Zyw2w@public.gmane.org>]

On Mon, 19 Nov 2007 03:44:51 am Amit Shah wrote:
> >From 76204eda7e03035c16702105e78724137ecad24b Mon Sep 17 00:00:00 2001
>
> From: Amit Shah <amit.shah-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
> Date: Sun, 18 Nov 2007 22:42:47 +0530
> Subject: [PATCH] KVM: SVM: Disable Lazy FPU optimization because of
> regressions
>
> Host FPU state is leaked into the guest FPU state. This happens
> because of the lazy FPU optimization, so just reload the FPU
> each time there's a VM exit/entry.
>
> The real fix should follow soon.
>
> This is observed in cases where fonts in a guest aren't rendered
> correctly (bug 1807560).
>
> The test program, courtesy Avi is:
>
> double test_fpu_once()
> {
>     int i;
>     double f = 0;
>
>     for (i = 0; i < 10000000; ++i)
>         f += 1 / (1.0 + i);
>     return f;
> }
>
> void test_fpu()
> {
>     double a, b;
>     int runs;
>
>     runs = 0;
>     a = test_fpu_once();
>     while (1) {
>         b = test_fpu_once();
>         if (fabs(a - b) > 1e-9)
>             printf("error: %20.16f -> %20.16f\n", a, b);
>         a = b;
>         if (++runs % 100 == 0)
>             printf("runs: %8d\n", runs);
>     }
> }
>
> int main(int ac, char **av)
> {
>     test_fpu();
>     return 0;
> }
>
> Run this on the host and the guest, both pinned to the same
> host CPU.
>
> Signed-off-by: Amit Shah <amit.shah-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
> ---
>  drivers/kvm/svm.c |   10 +++++-----
>  1 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/kvm/svm.c b/drivers/kvm/svm.c
> index ea2cb83..d7c9b92 100644
> --- a/drivers/kvm/svm.c
> +++ b/drivers/kvm/svm.c
> @@ -1600,11 +1600,11 @@ static void svm_set_cr3(struct kvm_vcpu *vcpu,
> unsigned long root)
>  	svm->vmcb->save.cr3 = root;
>  	force_new_asid(vcpu);
>
> -	if (vcpu->fpu_active) {
> -		svm->vmcb->control.intercept_exceptions |= (1 << NM_VECTOR);
> -		svm->vmcb->save.cr0 |= X86_CR0_TS;
> -		vcpu->fpu_active = 0;
> -	}
> +/* 	if (vcpu->fpu_active) { */
> +/* 		svm->vmcb->control.intercept_exceptions |= (1 << NM_VECTOR); */
> +/* 		svm->vmcb->save.cr0 |= X86_CR0_TS; */
> +/* 		vcpu->fpu_active = 0; */
> +/* 	} */
>  }
>
>  static void svm_inject_page_fault(struct kvm_vcpu *vcpu,

This patch didnt resolve the font issues for me, however the other patch 
(unload FPU state) did fix it.  I didnt run the test app (no compiler on the 
(32bit ubuntu) guest atm) but reverting this patch on my tree didnt appear to 
adversely affect anything.  Are both patches necessary?

Mike

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