RE: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly

Tian, Kevin
2021-12-11T03:07:48Z

> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Saturday, December 11, 2021 8:11 AM
>
> On Tue, Dec 07 2021 at 19:03, Yang Zhong wrote:
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index 5089f2e7dc22..9811dc98d550 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -238,6 +238,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest
> *gfpu)
> > fpstate->is_guest = true;
> >
> > gfpu->fpstate = fpstate;
> > + gfpu->xfd_err = XFD_ERR_GUEST_DISABLED;
>
> This wants to be part of the previous patch, which introduces the field.
>
> > gfpu->user_xfeatures = fpu_user_cfg.default_features;
> > gfpu->user_perm = fpu_user_cfg.default_features;
> > fpu_init_guest_permissions(gfpu);
> > @@ -297,6 +298,7 @@ int fpu_swap_kvm_fpstate(struct fpu_guest
> *guest_fpu, bool enter_guest)
> > fpu->fpstate = guest_fps;
> > guest_fps->in_use = true;
> > } else {
> > + fpu_save_guest_xfd_err(guest_fpu);
>
> Hmm. See below.
>
> > guest_fps->in_use = false;
> > fpu->fpstate = fpu->__task_fpstate;
> > fpu->__task_fpstate = NULL;
> > @@ -4550,6 +4550,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > kvm_steal_time_set_preempted(vcpu);
> > srcu_read_unlock(&vcpu->kvm->srcu, idx);
> >
> > + if (vcpu->preempted)
> > + fpu_save_guest_xfd_err(&vcpu->arch.guest_fpu);
>
> I'm not really exited about the thought of an exception cause register
> in guest clobbered state.
>
> Aside of that I really have to ask the question why all this is needed?
>
> #NM in the guest is slow path, right? So why are you trying to optimize
> for it?

This is really good information. The current logic is obviously
based on the assumption that #NM is frequently triggered.

>
> The straight forward solution to this is:
>
> 1) Trap #NM and MSR_XFD_ERR write

and #NM vmexit handler should be called in kvm_x86_handle_exit_irqoff()
before preemption is enabled, otherwise there is still a small window
where MSR_XFD_ERR might be clobbered after preemption enable and
before #NM handler is actually called.

>
> 2) When the guest triggers #NM is takes an VMEXIT and the host
> does:
>
> rdmsrl(MSR_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>
> injects the #NM and goes on.
>
> 3) When the guest writes to MSR_XFD_ERR it takes an VMEXIT and
> the host does:
>
> vcpu->arch.guest_fpu.xfd_err = msrval;
> wrmsrl(MSR_XFD_ERR, msrval);
>
> and goes back.
>
> 4) Before entering the preemption disabled section of the VCPU loop
> do:
>
> if (vcpu->arch.guest_fpu.xfd_err)
> wrmsrl(MSR_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>
> 5) Before leaving the preemption disabled section of the VCPU loop
> do:
>
> if (vcpu->arch.guest_fpu.xfd_err)
> wrmsrl(MSR_XFD_ERR, 0);
>
> It's really that simple and pretty much 0 overhead for the regular case.

Much cleaner.

>
> If the guest triggers #NM with a high frequency then taking the VMEXITs
> is the least of the problems. That's not a realistic use case, really.
>
> Hmm?
>
> Thanks,
>
> tglx

Thanks
Kevin