2017-11-15 22:40 GMT+08:00 Rik van Riel <riel@xxxxxxxxxx>: > On Wed, 2017-11-15 at 12:33 +0800, Wanpeng Li wrote: >> 2017-11-15 11:03 GMT+08:00 Rik van Riel <riel@xxxxxxxxxx>: >> > On Wed, 2017-11-15 at 08:47 +0800, Wanpeng Li wrote: >> > > 2017-11-15 5:54 GMT+08:00 <riel@xxxxxxxxxx>: >> > > > From: Rik van Riel <riel@xxxxxxxxxx> >> > > > >> > > > Currently, every time a VCPU is scheduled out, the host kernel >> > > > will >> > > > first save the guest FPU/xstate context, then load the qemu >> > > > userspace >> > > > FPU context, only to then immediately save the qemu userspace >> > > > FPU >> > > > context back to memory. When scheduling in a VCPU, the same >> > > > extraneous >> > > > FPU loads and saves are done. >> > > > >> > > > This could be avoided by moving from a model where the guest >> > > > FPU is >> > > > loaded and stored with preemption disabled, to a model where >> > > > the >> > > > qemu userspace FPU is swapped out for the guest FPU context for >> > > > the duration of the KVM_RUN ioctl. >> > > >> > > What will happen if CONFIG_PREEMPT is enabled? >> > >> > The scheduler will save the guest FPU context when a >> > VCPU thread is preempted, and restore it when it is >> > scheduled back in. >> >> I mean all the involved processes will use fpu. Before patch if >> kernel >> preempt occur: >> >> context_switch >> -> prepare_task_switch >> -> fire_sched_out_preempt_notifiers >> -> kvm_sched_out >> -> kvm_arch_vcpu_put >> -> kvm_put_guest_fpu >> -> copy_fpregs_to_fpstate(&vcpu- >> >arch.guest_fpu) >> save xsave area to guest fpu >> buffer >> -> __kernel_fpu_end >> -> >> copy_kernel_to_fpregs(¤t->thread.fpu.state) >> restore prev vCPU qemu >> userspace FPU to the xsave area >> -> switch_to >> -> __switch_to >> -> switch_fpu_prepare >> -> copy_fpregs_to_fpstate => save xsave area to >> prev >> vCPU qemu userspace FPU >> -> switch_fpu_finish >> -> copy_kernel_to_fpgregs => restore next task FPU >> to xsave area >> >> >> After the patch: >> >> context_switch >> -> prepare_task_switch >> -> fire_sched_out_preempt_notifiers >> -> kvm_sched_out >> >> -> switch_to >> -> __switch_to >> -> switch_fpu_prepare >> -> copy_fpregs_to_fpstate => Oops >> save xsave area to prev vCPU qemu userspace FPU, >> actually the guest FPU buffer is loaded in xsave area, you transmit >> guest FPU in xsave area into the prev vCPU qemu userspace FPU > > When entering kvm_arch_vcpu_ioctl_run we save the qemu userspace > FPU context in &vcpu->arch.user_fpu, and we restore that before > leaving kvm_arch_vcpu_ioctl_run. > > Userspace should always see the userspace FPU context, no? You are right. Regards, Wanpeng Li
