Re: [PATCH RFC: not fully tested yet] Fix the x86 emulation of in: remove redandancy

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

Takuya Yoshikawa wrote:
> Fix the x86 emulation of in:
> 
> kvm_emulate_pio() and complete_pio() both read out the
> RAX register value and copy it to a place into which
> the value read out from the port will be copied later.
> This patch removes this redundancy.
> 
> /*** snippet from arch/x86/kvm/x86.c ***/
> int complete_pio(struct kvm_vcpu *vcpu)
> {
> 	...
> 	if (!io->string) {
> 		if (io->in) {
> 			val = kvm_register_read(vcpu, VCPU_REGS_RAX);
> 			memcpy(&val, vcpu->arch.pio_data, io->size);
> 			kvm_register_write(vcpu, VCPU_REGS_RAX, val);
> 		}
> 	...
> 

Self explanation: reading RAX before copying pio data is for preserving
higher bits of the register. If we are sure that pio emulation ends by
complete_pio() and complete_pio() does this higher bits preservation,
we no longer need to mind preserving the higer bits of arch.pio_data
in each device's read handler: may reduce some switches and memcpies,
if we want.

Anyway, if no device's read handler depends on the memcpied 4 bytes of
RAX, I hope so, this patch seems to work and reduces extra memcpy.

> Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> ---
>  arch/x86/kvm/x86.c |    6 ++++--
>  1 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d47ceda..fcbe3a7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3550,8 +3550,10 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, int in, int size, unsigned port)
>  	trace_kvm_pio(vcpu->run->io.direction == KVM_EXIT_IO_OUT, port,
>  		      size, 1);
>  
> -	val = kvm_register_read(vcpu, VCPU_REGS_RAX);
> -	memcpy(vcpu->arch.pio_data, &val, 4);
> +	if (!vcpu->arch.pio.in) {
> +		val = kvm_register_read(vcpu, VCPU_REGS_RAX);
> +		memcpy(vcpu->arch.pio_data, &val, 4);
> +	}
>  
>  	if (!kernel_pio(vcpu, vcpu->arch.pio_data)) {
>  		complete_pio(vcpu);