Re: [RFC PATCH 2/2] KVM: x86: use __kvm_guest_exit

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/06/2016 18:43, David Matlack wrote:
> 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?

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.

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

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
>>