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(¤t->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
