From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755941AbdKNQ6P (ORCPT ); Tue, 14 Nov 2017 11:58:15 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57820 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755918AbdKNQ55 (ORCPT ); Tue, 14 Nov 2017 11:57:57 -0500 Subject: Re: [PATCH] x86,kvm: move qemu/guest FPU switching out to vcpu_run To: Rik van Riel , pbonzini@redhat.com Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, rkrcmar@redhat.com, borntraeger@de.ibm.com References: <20171114001223.441ea2ca@annuminas.surriel.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Tue, 14 Nov 2017 17:57:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171114001223.441ea2ca@annuminas.surriel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 14 Nov 2017 16:57:57 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14.11.2017 06:12, Rik van Riel wrote: > 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. > > 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 > Suggested-by: Christian Borntraeger > --- > arch/x86/include/asm/kvm_host.h | 13 +++++++++++++ > arch/x86/kvm/x86.c | 29 ++++++++++++----------------- > 2 files changed, 25 insertions(+), 17 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index c73e493adf07..92e66685249e 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h We should also get rid of guest_fpu_loaded now, right? > @@ -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..59912b20a830 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(); > } > > @@ -6908,7 +6907,6 @@ 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 > @@ -7095,6 +7093,8 @@ static int vcpu_run(struct kvm_vcpu *vcpu) > > vcpu->srcu_idx = srcu_read_lock(&kvm->srcu); > > + kvm_load_guest_fpu(vcpu); > + > for (;;) { > if (kvm_vcpu_running(vcpu)) { > r = vcpu_enter_guest(vcpu); > @@ -7132,6 +7132,8 @@ static int vcpu_run(struct kvm_vcpu *vcpu) > } > } > > + kvm_put_guest_fpu(vcpu); > + > srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx); > > return r; > @@ -7663,32 +7665,25 @@ 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); > } > emulator_get_fpu() does a kvm_load_guest_fpu(). Doesn't that mean that this is now not needed anymore? (at least when emulator code is called from inside the loop?) Also, what about preempt_diable() at that point, still needed? -- Thanks, David / dhildenb