Re: [PATCH] Revert "kvm: x86: Use task structs fpu field for user"

[Wanpeng Li]

On Mon, 22 Jul 2019 at 20:49, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> This reverts commit 240c35a3783ab9b3a0afaba0dde7291295680a6b
> ("kvm: x86: Use task structs fpu field for user", 2018-11-06).
> The commit is broken and causes QEMU's FPU state to be destroyed
> when KVM_RUN is preempted.
>
> Fixes: 240c35a3783a ("kvm: x86: Use task structs fpu field for user")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h | 7 ++++---
>  arch/x86/kvm/x86.c              | 4 ++--
>  2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0cc5b611a113..b2f1ffb937af 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -607,15 +607,16 @@ struct kvm_vcpu_arch {
>
>         /*
>          * QEMU userspace and the guest each have their own FPU state.
> -        * In vcpu_run, we switch between the user, maintained in the
> -        * task_struct struct, and guest FPU contexts. While running a VCPU,
> -        * the VCPU thread will have the guest FPU context.
> +        * In vcpu_run, we switch between the user and guest FPU contexts.
> +        * While running a VCPU, the VCPU thread will have the guest FPU
> +        * context.
>          *
>          * Note that while the PKRU state lives inside the fpu registers,
>          * it is switched out separately at VMENTER and VMEXIT time. The
>          * "guest_fpu" state here contains the guest FPU context, with the
>          * host PRKU bits.
>          */
> +       struct fpu user_fpu;
>         struct fpu *guest_fpu;
>
>         u64 xcr0;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 58305cf81182..cf2afdf8facf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8270,7 +8270,7 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
>  {
>         fpregs_lock();
>
> -       copy_fpregs_to_fpstate(&current->thread.fpu);
> +       copy_fpregs_to_fpstate(&vcpu->arch.user_fpu);
>         /* PKRU is separately restored in kvm_x86_ops->run.  */
>         __copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
>                                 ~XFEATURE_MASK_PKRU);
> @@ -8287,7 +8287,7 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>         fpregs_lock();
>
>         copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
> -       copy_kernel_to_fpregs(&current->thread.fpu.state);
> +       copy_kernel_to_fpregs(&vcpu->arch.user_fpu.state);
>
>         fpregs_mark_activate();
>         fpregs_unlock();

Looks good to me.

Regards,
Wanpeng Li