linux-kernel - Re: [PATCH] KVM: Reduce the execution of one instruction

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Message-ID: <Yxdx36BHlClCq52J@google.com>
Date:   Tue, 6 Sep 2022 16:14:23 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Liam Ni <zhiguangni01@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org,
        pbonzini@...hat.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com
Subject: Re: [PATCH] KVM: Reduce the execution of one instruction

"KVM: x86:" for the shortlog.

On Tue, Sep 06, 2022, Liam Ni wrote:
> From: Liam Ni <zhiguangni01@...il.com>
> 
> If the condition is met,

Please describe this specific code change, "If the condition is met" is extremely
generic and doesn't help the reader understand what change is being made.

> reduce the execution of one instruction.

This is highly speculative, e.g. clang will generate identical output since it's
trivial for the compiler to observe that ctxt->modrm_reg doesn't need to be read.

And similar to the above "If the condition is met", the shortlog is too generic
even if it were 100% accurate.

I do think this change is a net positive, but it's beneficial only in making the
code easier to read.  Shaving a single cheap instruction in a relatively slow path
isn't sufficient justification even if the compiler isn't clever enough to optimize
away the load in the first place.

E.g. something like:

  KVM: x86: Clean up ModR/M "reg" initialization in reg op decoding

  Refactor decode_register_operand() to get the ModR/M register if and
  only if the instruction uses a ModR/M encoding to make it more obvious
  how the register operand is retrieved.

> Signed-off-by: Liam Ni <zhiguangni01@...il.com>
> ---
>  arch/x86/kvm/emulate.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index f8382abe22ff..ebb95f3f9862 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1139,10 +1139,12 @@ static int em_fnstsw(struct x86_emulate_ctxt *ctxt)
>  static void decode_register_operand(struct x86_emulate_ctxt *ctxt,
>  				    struct operand *op)
>  {
> -	unsigned reg = ctxt->modrm_reg;
> +	unsigned int reg;
>  
>  	if (!(ctxt->d & ModRM))
>  		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);
> +	else
> +		reg = ctxt->modrm_reg;

I'd prefer to write this as:

	unsigned int reg;

	if (ctxt->d & ModRM)
		reg = ctxt->modrm_reg;
	else
		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);

so that "is ModRM" check is immediately followed by "get ModRM".

>  
>  	if (ctxt->d & Sse) {
>  		op->type = OP_XMM;
> -- 
> 2.25.1

>