Re: [RFC PATCH 2/5] KVM: x86: avoid useless set of KVM_REQ_EVENT after emulation

[Paolo Bonzini]

Il 27/03/2014 12:30, Paolo Bonzini ha scritto:
> Despite the provisions to emulate up to 130 consecutive instructions, in
> practice KVM will emulate just one before exiting handle_invalid_guest_state,
> because x86_emulate_instructionn always sets KVM_REQ_EVENT.
> 
> However, we only need to do this if an interrupt could be injected,
> which happens a) if an interrupt shadow bit (STI or MOV SS) has gone
> away; b) if the interrupt flag has just been set (because other
> instructions than STI can set it without enabling an interrupt shadow).
> 
> This cuts another 250-300 clock cycles from the cost of emulating an
> instruction (530-870 cycles before the patch on kvm-unit-tests,
> 290-600 afterwards).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fd31aada351b..ce9523345f2e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -87,6 +87,7 @@ static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
>  
>  static void update_cr8_intercept(struct kvm_vcpu *vcpu);
>  static void process_nmi(struct kvm_vcpu *vcpu);
> +static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
>  
>  struct kvm_x86_ops *kvm_x86_ops;
>  EXPORT_SYMBOL_GPL(kvm_x86_ops);
> @@ -4856,8 +4857,10 @@ static void toggle_interruptibility(struct kvm_vcpu *vcpu, u32 mask)
>  	 * means that the last instruction is an sti. We should not
>  	 * leave the flag on in this case. The same goes for mov ss
>  	 */
> -	if (!(int_shadow & mask))
> +	if (unlikely(int_shadow) && !(int_shadow & mask)) {
>  		kvm_x86_ops->set_interrupt_shadow(vcpu, mask);
> +		kvm_make_request(KVM_REQ_EVENT, vcpu);
> +	}

Better:

 	 * means that the last instruction is an sti. We should not
 	 * leave the flag on in this case. The same goes for mov ss
 	 */
-	if (!(int_shadow & mask))
+	mask &= ~int_shadow;
+	if (unlikely(mask != int_shadow))
 		kvm_x86_ops->set_interrupt_shadow(vcpu, mask);
+
+	/*
+	 * The interrupt window might have opened if a bit has been cleared.
+	 */
+	if (unlikely(int_shadow & ~mask))
+		kvm_make_request(KVM_REQ_EVENT, vcpu);

Paolo

>  }
>  
>  static void inject_emulated_exception(struct kvm_vcpu *vcpu)
> @@ -5083,20 +5086,18 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
>  	return dr6;
>  }
>  
> -static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, int *r)
> +static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r)
>  {
>  	struct kvm_run *kvm_run = vcpu->run;
>  
>  	/*
> -	 * Use the "raw" value to see if TF was passed to the processor.
> -	 * Note that the new value of the flags has not been saved yet.
> +	 * rflags is the old, "raw" value of the flags.  The new value has
> +	 * not been saved yet.
>  	 *
>  	 * This is correct even for TF set by the guest, because "the
>  	 * processor will not generate this exception after the instruction
>  	 * that sets the TF flag".
>  	 */
> -	unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
> -
>  	if (unlikely(rflags & X86_EFLAGS_TF)) {
>  		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
>  			kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1;
> @@ -5263,13 +5264,15 @@ restart:
>  		r = EMULATE_DONE;
>  
>  	if (writeback) {
> +		unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
>  		toggle_interruptibility(vcpu, ctxt->interruptibility);
> -		kvm_make_request(KVM_REQ_EVENT, vcpu);
>  		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
>  		kvm_rip_write(vcpu, ctxt->eip);
>  		if (r == EMULATE_DONE)
> -			kvm_vcpu_check_singlestep(vcpu, &r);
> -		kvm_set_rflags(vcpu, ctxt->eflags);
> +			kvm_vcpu_check_singlestep(vcpu, rflags, &r);
> +		__kvm_set_rflags(vcpu, ctxt->eflags);
> +		if (unlikely((ctxt->eflags & ~rflags) & X86_EFLAGS_IF))
> +			kvm_make_request(KVM_REQ_EVENT, vcpu);
>  	} else
>  		vcpu->arch.emulate_regs_need_sync_to_vcpu = true;
>  
> @@ -7385,12 +7388,17 @@ unsigned long kvm_get_rflags(struct kvm_vcpu *vcpu)
>  }
>  EXPORT_SYMBOL_GPL(kvm_get_rflags);
>  
> -void kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
> +static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  {
>  	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP &&
>  	    kvm_is_linear_rip(vcpu, vcpu->arch.singlestep_rip))
>  		rflags |= X86_EFLAGS_TF;
>  	kvm_x86_ops->set_rflags(vcpu, rflags);
> +}
> +
> +void kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
> +{
> +	__kvm_set_rflags(vcpu, rflags);
>  	kvm_make_request(KVM_REQ_EVENT, vcpu);
>  }
>  EXPORT_SYMBOL_GPL(kvm_set_rflags);
> 

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