Re: [PATCH 1/4] KVM: x86: Handle TIF_NEED_FPU_LOAD in kvm_{load,put}_guest_fpu()

Sean Christopherson
2020-01-17T18:43:39Z

On Fri, Jan 17, 2020 at 10:31:53AM -0800, Dave Hansen wrote:
> On 1/16/20 10:26 PM, Sean Christopherson wrote:
> > Handle TIF_NEED_FPU_LOAD similar to how fpu__copy() handles the flag
> > when duplicating FPU state to a new task struct. TIF_NEED_FPU_LOAD can
> > be set any time control is transferred out of KVM, be it voluntarily,
> > e.g. if I/O is triggered during a KVM call to get_user_pages, or
> > involuntarily, e.g. if softirq runs after an IRQ occurs. Therefore,
> > KVM must account for TIF_NEED_FPU_LOAD whenever it is (potentially)
> > accessing CPU FPU state.
> >
> > Fixes: 5f409e20b7945 ("x86/fpu: Defer FPU state load until return to userspace")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> > ---
> > arch/x86/kvm/x86.c | 27 ++++++++++++++++++++++++---
> > 1 file changed, 24 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index cf917139de6b..0c7211491f98 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8476,8 +8476,20 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> > {
> > fpregs_lock();
> >
> > - copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
> > - /* PKRU is separately restored in kvm_x86_ops->run. */
> > + /*
> > + * If userspace's FPU state is not resident in the CPU registers, just
> > + * memcpy() from current, else save CPU state directly to user_fpu.
> > + */
> > + if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > + memcpy(&vcpu->arch.user_fpu->state, &current->thread.fpu.state,
> > + fpu_kernel_xstate_size);
> > + else
> > + copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
> > +
> > + /*
> > + * Load guest's FPU state to the CPU registers. PKRU is separately
> > + * loaded in kvm_x86_ops->run.
> > + */
> > __copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
> > ~XFEATURE_MASK_PKRU);
>
> Nit: it took me a minute to realize that there is both:
>
> vcpu->arch.user_fpu
> and
> vcpu->arch.guest_fpu
>
> It might help readability to have local variables for those, or at least
> a comment to help differentiate the two.

Or even better, add a helper to wrap the logic instead of copy+paste, e.g.:

static void kvm_save_current_fpu(struct fpu *fpu)
{
if (test_thread_flag(TIF_NEED_FPU_LOAD))
memcpy(&fpu->state, &current->thread.fpu.state,
fpu_kernel_xstate_size);
else
copy_fpregs_to_fpstate(fpu);
}

>
>
> > @@ -8492,7 +8504,16 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> > {
> > fpregs_lock();
> >
> > - copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
> > + /*
> > + * If guest's FPU state is not resident in the CPU registers, just
> > + * memcpy() from current, else save CPU state directly to guest_fpu.
> > + */
> > + if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > + memcpy(&vcpu->arch.guest_fpu->state, &current->thread.fpu.state,
> > + fpu_kernel_xstate_size);
> > + else
> > + copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
> > +
> > copy_kernel_to_fpregs(&vcpu->arch.user_fpu->state);
> >
> > fpregs_mark_activate();
>
> This also makes me wonder if we want to have copy_fpregs_to_fpstate()
> check for TIF_NEED_FPU_LOAD and complain if it's set.