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SubjectRe: [RFC PATCH 2/2] KVM: x86: use __kvm_guest_exit
On Thu, Jun 16, 2016 at 1:21 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> This gains ~20 clock cycles per vmexit. On Intel there is no need
> anymore to enable the interrupts in vmx_handle_external_intr, since we
> are using the "acknowledge interrupt on exit" feature. AMD needs to do
> that temporarily, and must be careful to avoid the interrupt shadow.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/svm.c | 6 ++++++
> arch/x86/kvm/vmx.c | 4 +---
> arch/x86/kvm/x86.c | 11 ++---------
> 3 files changed, 9 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 5ff292778110..5bfdbbf1ce79 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4935,6 +4935,12 @@ out:
> static void svm_handle_external_intr(struct kvm_vcpu *vcpu)
> {
> local_irq_enable();
> + /*
> + * We must execute an instruction with interrupts enabled, so
> + * the "cli" doesn't fall right on the interrupt shadow.
> + */
> + asm("nop");
> + local_irq_disable();
> }
>
> static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 4e9657730bf6..a46bce9e3683 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -8544,7 +8544,6 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
> "push %[sp]\n\t"
> #endif
> "pushf\n\t"
> - "orl $0x200, (%%" _ASM_SP ")\n\t"
> __ASM_SIZE(push) " $%c[cs]\n\t"
> "call *%[entry]\n\t"
> :
> @@ -8557,8 +8556,7 @@ static void vmx_handle_external_intr(struct kvm_vcpu *vcpu)
> [ss]"i"(__KERNEL_DS),
> [cs]"i"(__KERNEL_CS)
> );
> - } else
> - local_irq_enable();
> + }

If you make the else case the same as svm_handle_external_intr, can we
avoid requiring ack-intr-on-exit?

> }
>
> static bool vmx_has_high_real_mode_segbase(void)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7e3041ef050f..cc741b68139c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6706,21 +6706,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>
> kvm_put_guest_xcr0(vcpu);
>
> - /* Interrupt is enabled by handle_external_intr() */
> kvm_x86_ops->handle_external_intr(vcpu);
>
> ++vcpu->stat.exits;
>
> - /*
> - * We must have an instruction between local_irq_enable() and
> - * kvm_guest_exit(), so the timer interrupt isn't delayed by
> - * the interrupt shadow. The stat.exits increment will do nicely.
> - * But we need to prevent reordering, hence this barrier():
> - */
> - barrier();
> -
> - kvm_guest_exit();
> + __kvm_guest_exit();
>
> + local_irq_enable();
> preempt_enable();
>
> vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> --
> 1.8.3.1
>

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