On 15.11.2017 15:50, Rik van Riel wrote: > On Wed, 2017-11-15 at 09:34 +0100, Paolo Bonzini wrote: >> On 14/11/2017 20:40, David Hildenbrand wrote: >>> I think we should check all get/put_fpu callers if they need >>> preempt_disable(). >>> >>> E.g. em_fxrstor() needs disabled preemption as we temporarily >>> save + restore some host register (via fxsave + fxrstor) under some >>> circumstances that are not saved/restored when switching to/back >>> from >>> another process. We should double check. >> >> Rik may correct me, but I believe that you don't need >> preempt_disable/enable because preempt notifiers do this for you. > > We no longer even need the preempt notifiers to save and > restore the guest FPU state. > > The context switch code itself will save the FPU state > from the registers, into current->thread.fpu.state, when > the VCPU thread gets scheduled out. > > When the VCPU thread gets scheduled in, the scheduler > will restore the guest FPU state from current->thread.fpu.state. > > At this point, vcpu->arch.guest_fpu may be OUT OF DATE. > > However, this is just fine, because we will save the guest > FPU state into vcpu->arch.guest_fpu in kvm_put_guest_fpu, > before we leave the KVM_RUN ioctl, and before we release > the vcpu->mutex. > > In other words, by the time anybody else can examine the > VCPU FPU state (after they obtain the vcpu->mutex), the > vcpu->arch.guest_fpu area will contain the correct FPU > state. > Okay, that answers my question, thanks! -- Thanks, David / dhildenb
