KVM: avoid unnecessary memcpy in kvm_emulate_pio

KVM: avoid unnecessary memcpy in kvm_emulate_pio

[Marcelo Tosatti]

When emulating in instructions there's no point in copying the
destination register contents to vcpu->arch.pio_data since it will be
overwritten by the ioport handler.

Also remove the ->decache_regs() call, since as you mentioned there is
no register change to writeback.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e537005..f43f770 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2254,8 +2254,8 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, struct kvm_run *run, int in,
 			    handler);
 
 	kvm_x86_ops->cache_regs(vcpu);
-	memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
-	kvm_x86_ops->decache_regs(vcpu);
+	if (!in)
+		memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
 
 	kvm_x86_ops->skip_emulated_instruction(vcpu);
 

Re: KVM: avoid unnecessary memcpy in kvm_emulate_pio

[Avi Kivity]

Marcelo Tosatti wrote:
> When emulating in instructions there's no point in copying the
> destination register contents to vcpu->arch.pio_data since it will be
> overwritten by the ioport handler.
>
> Also remove the ->decache_regs() call, since as you mentioned there is
> no register change to writeback.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e537005..f43f770 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2254,8 +2254,8 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, struct kvm_run *run, int in,
>  			    handler);
>  
>  	kvm_x86_ops->cache_regs(vcpu);
> -	memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
> -	kvm_x86_ops->decache_regs(vcpu);
> +	if (!in)
> +		memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
>  
>  	kvm_x86_ops->skip_emulated_instruction(vcpu);
>  

The memcpy() compiles to just one mov instruction, so I didn't change 
it.  I dropped the ->decache_regs() call (as it can take 100 cycles on 
Intel).

-- 
error compiling committee.c: too many arguments to function