linux-kernel - Re: [PATCH v2] KVM:x86: Clean up ModR/M "reg" initialization in reg op decoding

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Date:   Thu, 8 Sep 2022 15:50:35 +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 v2] KVM:x86: Clean up ModR/M "reg" initialization in reg
 op decoding

On Thu, Sep 08, 2022, Liam Ni wrote:
> From: Liam Ni <zhiguangni01@...il.com>
> 
> 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>
> ---

Pushed to branch `for_paolo/6.1` at:

    https://github.com/sean-jc/linux.git

with the below nit sqaushed.  Unless you hear otherwise, it will make its way to
kvm/queue "soon".

Note, the commit IDs are not guaranteed to be stable.

>  arch/x86/kvm/emulate.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index f092c54d1a2f..879b52af763a 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1137,9 +1137,11 @@ 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))
> +	if ((ctxt->d & ModRM))

Only need one set of parentheses.  

> +		reg = ctxt->modrm_reg;
> +	else
>  		reg = (ctxt->b & 7) | ((ctxt->rex_prefix & 1) << 3);
>  
>  	if (ctxt->d & Sse) {
> -- 
> 2.34.1
>