linux-kernel - Re: [RFC PATCH 2/2] KVM: x86: use __kvm_guest

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Date:	Thu, 16 Jun 2016 09:43:54 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>, bsd@...hat.com,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [RFC PATCH 2/2] KVM: x86: use __kvm_guest_exit

On Thu, Jun 16, 2016 at 1:21 AM, Paolo Bonzini <pbonzini@...hat.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@...hat.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
>