On Tue, Mar 10, 2020 at 03:01:19PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> kvm_load_guest_fpu() and kvm_put_guest_fpu() each consume more than 14us
> observed by ftrace, the qemu userspace FPU is swapped out for the guest
> FPU context for the duration of the KVM_RUN ioctl even if sleeping AP,
> we shouldn't load/put guest FPU context for this case especially for
> serverless scenario which sensitives to boot time.
>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5de2006..080ffa4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8680,7 +8680,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>
> vcpu_load(vcpu);
> kvm_sigset_activate(vcpu);
> - kvm_load_guest_fpu(vcpu);
>
> if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
> if (kvm_run->immediate_exit) {
> @@ -8718,12 +8717,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> }
> }
>
> + kvm_load_guest_fpu(vcpu);
Ugh, so this isn't safe on MPX capable CPUs, kvm_apic_accept_events() can
trigger kvm_vcpu_reset() with @init_event=true and try to unload guest_fpu.
We could hack around that issue, but it'd be ugly, and I'm also concerned
that calling vmx_vcpu_reset() without guest_fpu loaded will be problematic
in the future with all the things that are getting managed by XSAVE.
> +
> if (unlikely(vcpu->arch.complete_userspace_io)) {
> int (*cui)(struct kvm_vcpu *) = vcpu->arch.complete_userspace_io;
> vcpu->arch.complete_userspace_io = NULL;
> r = cui(vcpu);
> if (r <= 0)
> - goto out;
> + goto out_fpu;
> } else
> WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
>
> @@ -8732,8 +8733,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> else
> r = vcpu_run(vcpu);
>
> -out:
> +out_fpu:
> kvm_put_guest_fpu(vcpu);
> +out:
> if (vcpu->run->kvm_valid_regs)
> store_regs(vcpu);
> post_kvm_run_save(vcpu);
> --
> 2.7.4
>
