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

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Date:	Thu, 16 Jun 2016 10:03:00 -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 9:47 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> On 16/06/2016 18:43, David Matlack wrote:
>> 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?
>
> Yes, but the sti/nop/cli would be useless if ack-intr-on-exit is
> available.  It's a bit ugly, so I RFCed the bold thing instead.

Ahh, and handle_external_intr is called on every VM-exit, not just
VM-exits caused by external interrupts. So we'd be doing the
sti/nop/cli quite often. I was thinking we never hit the else case
when the CPU supports ack-intr-on-exit.

>
> Are you thinking of some distros in particular that lack nested
> ack-intr-on-exit?  All processors have it as far as I know.

Nope, I just thought it was possible to avoid the requirement.

>
> Paolo
>
>
>>>  }
>>>
>>>  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
>>>