* [PATCH] KVM: Reduce the execution of one instruction
@ 2022-09-06 15:33 Liam Ni
2022-09-06 16:14 ` Sean Christopherson
0 siblings, 1 reply; 3+ messages in thread
From: Liam Ni @ 2022-09-06 15:33 UTC (permalink / raw)
To: linux-kernel, kvm, x86; +Cc: seanjc, pbonzini, tglx, dave.hansen, zhiguangni01
From: Liam Ni <zhiguangni01@gmail.com>
If the condition is met, reduce the execution of one instruction.
Signed-off-by: Liam Ni <zhiguangni01@gmail.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;
if (ctxt->d & Sse) {
op->type = OP_XMM;
--
2.25.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: Reduce the execution of one instruction
2022-09-06 15:33 [PATCH] KVM: Reduce the execution of one instruction Liam Ni
@ 2022-09-06 16:14 ` Sean Christopherson
2022-09-07 16:03 ` Liam Ni
0 siblings, 1 reply; 3+ messages in thread
From: Sean Christopherson @ 2022-09-06 16:14 UTC (permalink / raw)
To: Liam Ni; +Cc: linux-kernel, kvm, x86, pbonzini, tglx, dave.hansen
"KVM: x86:" for the shortlog.
On Tue, Sep 06, 2022, Liam Ni wrote:
> From: Liam Ni <zhiguangni01@gmail.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@gmail.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
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] KVM: Reduce the execution of one instruction
2022-09-06 16:14 ` Sean Christopherson
@ 2022-09-07 16:03 ` Liam Ni
0 siblings, 0 replies; 3+ messages in thread
From: Liam Ni @ 2022-09-07 16:03 UTC (permalink / raw)
To: Sean Christopherson; +Cc: linux-kernel, kvm, x86, pbonzini, tglx, dave.hansen
On Wed, 7 Sept 2022 at 00:14, Sean Christopherson <seanjc@google.com> wrote:
>
> "KVM: x86:" for the shortlog.
>
> On Tue, Sep 06, 2022, Liam Ni wrote:
> > From: Liam Ni <zhiguangni01@gmail.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@gmail.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
>
> >
Thanks for the suggestion, I will submit a new patch V2.
^ permalink raw reply [flat|nested] 3+ messages in thread
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