Re: [PATCH 2/5] KVM: x86: drop fpu_activate hook

[Avi Kivity]

On 08/18/2014 01:51 PM, Paolo Bonzini wrote:
> Il 18/08/2014 12:26, Avi Kivity ha scritto:
> > On 08/18/2014 01:20 PM, Paolo Bonzini wrote:
> > > Il 18/08/2014 11:50, Wanpeng Li ha scritto:
> > > > fpu_activate hook is introduced by commit 6b52d186 (KVM: Activate fpu on
> > > > clts), however, there is no user currently, this patch drop it.
> > > >
> > > > Reviewed-by: Yang Zhang <yang.z.zhang@xxxxxxxxx>
> > > > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
> > > > ---
> > > >    arch/x86/include/asm/kvm_host.h | 1 -
> > > >    arch/x86/kvm/svm.c              | 1 -
> > > >    arch/x86/kvm/vmx.c              | 1 -
> > > >    3 files changed, 3 deletions(-)
> > > >
> > > > diff --git a/arch/x86/include/asm/kvm_host.h
> > > > b/arch/x86/include/asm/kvm_host.h
> > > > index 5724601..b68f3e5 100644
> > > > --- a/arch/x86/include/asm/kvm_host.h
> > > > +++ b/arch/x86/include/asm/kvm_host.h
> > > > @@ -710,7 +710,6 @@ struct kvm_x86_ops {
> > > >        void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
> > > >        unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
> > > >        void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
> > > > -    void (*fpu_activate)(struct kvm_vcpu *vcpu);
> > > >        void (*fpu_deactivate)(struct kvm_vcpu *vcpu);
> > > >          void (*tlb_flush)(struct kvm_vcpu *vcpu);
> > > > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > > > index ddf7427..1f49c86 100644
> > > > --- a/arch/x86/kvm/svm.c
> > > > +++ b/arch/x86/kvm/svm.c
> > > > @@ -4349,7 +4349,6 @@ static struct kvm_x86_ops svm_x86_ops = {
> > > >        .cache_reg = svm_cache_reg,
> > > >        .get_rflags = svm_get_rflags,
> > > >        .set_rflags = svm_set_rflags,
> > > > -    .fpu_activate = svm_fpu_activate,
> > > >        .fpu_deactivate = svm_fpu_deactivate,
> > > >          .tlb_flush = svm_flush_tlb,
> > > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > > > index 71cbee5..2963303 100644
> > > > --- a/arch/x86/kvm/vmx.c
> > > > +++ b/arch/x86/kvm/vmx.c
> > > > @@ -8896,7 +8896,6 @@ static struct kvm_x86_ops vmx_x86_ops = {
> > > >        .cache_reg = vmx_cache_reg,
> > > >        .get_rflags = vmx_get_rflags,
> > > >        .set_rflags = vmx_set_rflags,
> > > > -    .fpu_activate = vmx_fpu_activate,
> > > >        .fpu_deactivate = vmx_fpu_deactivate,
> > > >          .tlb_flush = vmx_flush_tlb,
> > > Avi/Gleb, do you remember any particular reason for this?
> > IIRC (vaguely) if we expect the fpu to be used in the near future, we
> > activate it eagerly so that we don't fault when it is used.
> >
> > Prevents the sequence:
> >
> > guest user: use fpu
> > #NM
> > host: reflect #NM to guest
> > guest kernel: CLTS
> > guest kernel: switch fpu state
> > #NM
> > host: switch fpu
> > guest kernel: switch fpu state (restarted)
> > guest user: use fpu (restarted)
> >
> > Why was the user removed? Full-time eager fpu?
> No, I mean any reason to keep the hooks.

If there are no callers, I can't think of any.

> In the meanwhile I found it myself:
>
> commit 2d04a05bd7e93c13f13a82ac40de4065a99d069b
> Author: Avi Kivity <avi@xxxxxxxxxx>
> Date:   Wed Apr 20 15:32:49 2011 +0300
>
>      KVM: x86 emulator: emulate CLTS internally
>      
>      Avoid using ctxt->vcpu; we can do everything with ->get_cr() and ->set_cr().
>      
>      A side effect is that we no longer activate the fpu on emulated CLTS; but that
>      should be very rare.
>      
>      Signed-off-by: Avi Kivity <avi@xxxxxxxxxx>
>
> vmx_fpu_activate and svm_fpu_activate are still called on #NM and CLTS, but
> never from common code after the above patch.
>
> Activation on CLTS is currently VMX only; I guess on AMD we could check the
> decode assists' CR_VALID bit and instruction length to detect CLTS.


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