"KVM: x86:" for the shortlog.
On Tue, Sep 06, 2022, Liam Ni wrote:
> From: Liam Ni <zhiguangni01@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> ---
> 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
>
