Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run

From: Wanpeng Li
Date: Tue Nov 14 2017 - 19:48:06 EST

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?

Regards,
Wanpeng Li

>
> This is done under the VCPU mutex, which is also taken when other
> tasks inspect the VCPU FPU context, so the code should already be
> safe for this change. That should come as no surprise, given that
> s390 already has this optimization.
>
> No performance changes were detected in quick ping-pong tests on
> my 4 socket system, which is expected since an FPU+xstate load is
> on the order of 0.1us, while ping-ponging between CPUs is on the
> order of 20us, and somewhat noisy.
>
> There may be other tests where performance changes are noticeable.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> Suggested-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 13 +++++++++++++
> arch/x86/kvm/x86.c | 34 +++++++++++++---------------------
> include/linux/kvm_host.h | 2 +-
> 3 files changed, 27 insertions(+), 22 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9d7d856b2d89..ffe54958491f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -536,7 +536,20 @@ struct kvm_vcpu_arch {
> struct kvm_mmu_memory_cache mmu_page_cache;
> struct kvm_mmu_memory_cache mmu_page_header_cache;
>
> + /*
> + * QEMU userspace and the guest each have their own FPU state.
> + * In vcpu_run, we switch between the user and guest FPU contexts.
> + * While running a VCPU, the VCPU thread will have the guest FPU
> + * context.
> + *
> + * Note that while the PKRU state lives inside the fpu registers,
> + * it is switched out separately at VMENTER and VMEXIT time. The
> + * "guest_fpu" state here contains the guest FPU context, with the
> + * host PRKU bits.
> + */
> + struct fpu user_fpu;
> struct fpu guest_fpu;
> +
> u64 xcr0;
> u64 guest_supported_xcr0;
> u32 guest_xstate_size;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 03869eb7fcd6..aad5181ed4e9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2917,7 +2917,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> srcu_read_unlock(&vcpu->kvm->srcu, idx);
> pagefault_enable();
> kvm_x86_ops->vcpu_put(vcpu);
> - kvm_put_guest_fpu(vcpu);
> vcpu->arch.last_host_tsc = rdtsc();
> }
>
> @@ -5228,13 +5227,10 @@ static void emulator_halt(struct x86_emulate_ctxt *ctxt)
>
> static void emulator_get_fpu(struct x86_emulate_ctxt *ctxt)
> {
> - preempt_disable();
> - kvm_load_guest_fpu(emul_to_vcpu(ctxt));
> }
>
> static void emulator_put_fpu(struct x86_emulate_ctxt *ctxt)
> {
> - preempt_enable();
> }
>
> static int emulator_intercept(struct x86_emulate_ctxt *ctxt,
> @@ -6908,7 +6904,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> preempt_disable();
>
> kvm_x86_ops->prepare_guest_switch(vcpu);
> - kvm_load_guest_fpu(vcpu);
>
> /*
> * Disable IRQs before setting IN_GUEST_MODE. Posted interrupt
> @@ -7255,12 +7250,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> }
> }
>
> + kvm_load_guest_fpu(vcpu);
> +
> 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);
>
> @@ -7269,6 +7266,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
> else
> r = vcpu_run(vcpu);
>
> +out_fpu:
> + kvm_put_guest_fpu(vcpu);
> out:
> post_kvm_run_save(vcpu);
> if (vcpu->sigset_active)
> @@ -7663,32 +7662,25 @@ static void fx_init(struct kvm_vcpu *vcpu)
> vcpu->arch.cr0 |= X86_CR0_ET;
> }
>
> +/* Swap (qemu) user FPU context for the guest FPU context. */
> void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - if (vcpu->guest_fpu_loaded)
> - return;
> -
> - /*
> - * Restore all possible states in the guest,
> - * and assume host would use all available bits.
> - * Guest xcr0 would be loaded later.
> - */
> - vcpu->guest_fpu_loaded = 1;
> - __kernel_fpu_begin();
> + preempt_disable();
> + copy_fpregs_to_fpstate(&vcpu->arch.user_fpu);
> /* PKRU is separately restored in kvm_x86_ops->run. */
> __copy_kernel_to_fpregs(&vcpu->arch.guest_fpu.state,
> ~XFEATURE_MASK_PKRU);
> + preempt_enable();
> trace_kvm_fpu(1);
> }
>
> +/* When vcpu_run ends, restore user space FPU context. */
> void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> {
> - if (!vcpu->guest_fpu_loaded)
> - return;
> -
> - vcpu->guest_fpu_loaded = 0;
> + preempt_disable();
> copy_fpregs_to_fpstate(&vcpu->arch.guest_fpu);
> - __kernel_fpu_end();
> + copy_kernel_to_fpregs(&vcpu->arch.user_fpu.state);
> + preempt_enable();
> ++vcpu->stat.fpu_reload;
> trace_kvm_fpu(0);
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 6882538eda32..354608487b8d 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -232,7 +232,7 @@ struct kvm_vcpu {
> struct mutex mutex;
> struct kvm_run *run;
>
> - int guest_fpu_loaded, guest_xcr0_loaded;
> + int guest_xcr0_loaded;
> struct swait_queue_head wq;
> struct pid __rcu *pid;
> int sigset_active;
> --
> 2.9.4
>