Re: [PATCH] KVM: X86: Don't load/put guest FPU context for sleeping AP

[Wanpeng Li]

On Wed, 11 Mar 2020 at 00:01, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Tue, Mar 10, 2020 at 03:01:19PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > kvm_load_guest_fpu() and kvm_put_guest_fpu() each consume more than 14us
> > observed by ftrace, the qemu userspace FPU is swapped out for the guest
> > FPU context for the duration of the KVM_RUN ioctl even if sleeping AP,
> > we shouldn't load/put guest FPU context for this case especially for
> > serverless scenario which sensitives to boot time.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5de2006..080ffa4 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8680,7 +8680,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> >
> >       vcpu_load(vcpu);
> >       kvm_sigset_activate(vcpu);
> > -     kvm_load_guest_fpu(vcpu);
> >
> >       if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
> >               if (kvm_run->immediate_exit) {
> > @@ -8718,12 +8717,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> >               }
> >       }
> >
> > +     kvm_load_guest_fpu(vcpu);
>
> Ugh, so this isn't safe on MPX capable CPUs, kvm_apic_accept_events() can
> trigger kvm_vcpu_reset() with @init_event=true and try to unload guest_fpu.

Right.

>
> We could hack around that issue, but it'd be ugly, and I'm also concerned
> that calling vmx_vcpu_reset() without guest_fpu loaded will be problematic
> in the future with all the things that are getting managed by XSAVE.
>
> > +
> >       if (unlikely(vcpu->arch.complete_userspace_io)) {
> >               int (*cui)(struct kvm_vcpu *) = vcpu->arch.complete_userspace_io;
> >               vcpu->arch.complete_userspace_io = NULL;
> >               r = cui(vcpu);
> >               if (r <= 0)
> > -                     goto out;
> > +                     goto out_fpu;
> >       } else
> >               WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
> >
> > @@ -8732,8 +8733,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> >       else
> >               r = vcpu_run(vcpu);
> >
> > -out:
> > +out_fpu:
> >       kvm_put_guest_fpu(vcpu);
> > +out:
> >       if (vcpu->run->kvm_valid_regs)
> >               store_regs(vcpu);
> >       post_kvm_run_save(vcpu);
> > --
> > 2.7.4
> >