Re: [5.2 regression] x86/fpu changes cause crashes in KVM guest

[Wanpeng Li]

On Fri, 19 Jul 2019 at 19:09, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 19/07/19 10:59, Wanpeng Li wrote:
> > https://lkml.org/lkml/2017/11/14/891, "The scheduler will save the
> > guest fpu context when a vCPU thread is preempted, and restore it when
> > it is scheduled back in." But I can't find any scheduler codes do
> > this.
>
> That's because applying commit 240c35a37 was completely wrong.  The idea
> before commit 240c35a37 was that you have the following FPU states:
>
>                userspace (QEMU)             guest
> ---------------------------------------------------------------------------
>                processor                    vcpu->arch.guest_fpu
> >>> KVM_RUN: kvm_load_guest_fpu
>                vcpu->arch.user_fpu          processor
> >>> preempt out
>                vcpu->arch.user_fpu          current->thread.fpu
> >>> preempt in
>                vcpu->arch.user_fpu          processor
> >>> back to userspace
> >>> kvm_put_guest_fpu
>                processor                    vcpu->arch.guest_fpu
> ---------------------------------------------------------------------------
>
> After removing user_fpu, QEMU's FPU state is destroyed when KVM_RUN is
> preempted.  So that's already messed up (I'll send a revert), and given
> the diagram above your patch makes total sense.
>
> With the new lazy model we want to hook into kvm_vcpu_arch_load and get
> the state back to the processor from current->thread.fpu, and indeed
> switch_fpu_return is essentially copy_kernel_to_fpregs(&current->thread.
> fpu->state).
>
> However I would keep the fpregs_assert_state_consistent in
> kvm_arch_vcpu_load, and also
> WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD)) in vcpu_enter_guest.

Looks good to me, just send out two patches rebase on the revert.

Regards,
Wanpeng Li