Re: [PATCH 2/2] Provide fast path for "rep ins" emulation if possible.

[Avi Kivity <avi@redhat.com>]

On 05/23/2012 05:08 PM, Gleb Natapov wrote:
> "rep ins" emulation is going through emulator now. This is slow because
> emulator knows how to write back only one datum at a time. This patch
> provides fast path for the instruction in certain conditions. The
> conditions are: DF flag is not set, destination memory is RAM and single
> datum does not cross page boundary. If fast path code fails it falls
> back to emulation.
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index f75af40..20e3fb0 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1887,21 +1887,31 @@ static int io_interception(struct vcpu_svm *svm)
>  {
>  	struct kvm_vcpu *vcpu = &svm->vcpu;
>  	u32 io_info = svm->vmcb->control.exit_info_1; /* address size bug? */
> -	int size, in, string;
> +	int size, in, string, rep;
>  	unsigned port;
>  
>  	++svm->vcpu.stat.io_exits;
>  	string = (io_info & SVM_IOIO_STR_MASK) != 0;
> +	rep = (io_info & SVM_IOIO_REP_MASK) != 0;
>  	in = (io_info & SVM_IOIO_TYPE_MASK) != 0;
> -	if (string || in)
> -		return emulate_instruction(vcpu, 0) == EMULATE_DONE;
>  

If decode assists are not available, we still need to emulate, see 15.33.5.

>  	port = io_info >> 16;
>  	size = (io_info & SVM_IOIO_SIZE_MASK) >> SVM_IOIO_SIZE_SHIFT;
>  	svm->next_rip = svm->vmcb->control.exit_info_2;
> -	skip_emulated_instruction(&svm->vcpu);
>  
> -	return kvm_fast_pio_out(vcpu, size, port);
> +	if (!string && !in) {
> +		skip_emulated_instruction(&svm->vcpu);
> +		return kvm_fast_pio_out(vcpu, size, port);
> +	} else if (string && in && rep) {
> +		int addr_size = (io_info & SVM_IOIO_ASIZE_MASK) >>
> +			SVM_IOIO_ASIZE_SHIFT;
> +		int r = kvm_fast_string_pio_in(vcpu, size, port,
> +				ffs(addr_size) - 1);
> +		if (r != EMULATE_FAIL)
> +			return r == EMULATE_DONE;
> +	}
> +
> +	return emulate_instruction(vcpu, 0) == EMULATE_DONE;
>  }
>  
-- 
error compiling committee.c: too many arguments to function