* [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles.
@ 2020-12-10 8:15 Stephen Zhang
2020-12-10 9:56 ` Vitaly Kuznetsov
2020-12-12 0:34 ` Paolo Bonzini
0 siblings, 2 replies; 3+ messages in thread
From: Stephen Zhang @ 2020-12-10 8:15 UTC (permalink / raw)
To: pbonzini, seanjc, vkuznets, wanpengli, jmattson, joro, tglx,
mingo, bp, x86, hpa
Cc: kvm, linux-kernel, Stephen Zhang
code changes in handle_io() to save some CPU cycles.
Signed-off-by: Stephen Zhang <starzhangzsd@gmail.com>
---
arch/x86/kvm/vmx/vmx.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 47b8357..109bcf64 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4899,15 +4899,14 @@ static int handle_triple_fault(struct kvm_vcpu *vcpu)
static int handle_io(struct kvm_vcpu *vcpu)
{
unsigned long exit_qualification;
- int size, in, string;
+ int size, in;
unsigned port;
exit_qualification = vmx_get_exit_qual(vcpu);
- string = (exit_qualification & 16) != 0;
++vcpu->stat.io_exits;
- if (string)
+ if (exit_qualification & 16)
return kvm_emulate_instruction(vcpu, 0);
port = exit_qualification >> 16;
--
1.8.3.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles.
2020-12-10 8:15 [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles Stephen Zhang
@ 2020-12-10 9:56 ` Vitaly Kuznetsov
2020-12-12 0:34 ` Paolo Bonzini
1 sibling, 0 replies; 3+ messages in thread
From: Vitaly Kuznetsov @ 2020-12-10 9:56 UTC (permalink / raw)
To: Stephen Zhang
Cc: kvm, linux-kernel, Stephen Zhang, pbonzini, seanjc, wanpengli,
jmattson, joro, tglx, mingo, bp, x86, hpa
Stephen Zhang <starzhangzsd@gmail.com> writes:
> code changes in handle_io() to save some CPU cycles.
>
> Signed-off-by: Stephen Zhang <starzhangzsd@gmail.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 47b8357..109bcf64 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4899,15 +4899,14 @@ static int handle_triple_fault(struct kvm_vcpu *vcpu)
> static int handle_io(struct kvm_vcpu *vcpu)
> {
> unsigned long exit_qualification;
> - int size, in, string;
> + int size, in;
> unsigned port;
>
> exit_qualification = vmx_get_exit_qual(vcpu);
> - string = (exit_qualification & 16) != 0;
>
> ++vcpu->stat.io_exits;
>
> - if (string)
> + if (exit_qualification & 16)
> return kvm_emulate_instruction(vcpu, 0);
>
> port = exit_qualification >> 16;
I seriously doubt this can save any CPU cycles as compiler will likely
re-order and optimize this check anyway, I don't expect to see any
push/pop operations here. I agree that having local 'string' variable is
likely an overkill, however, the 'exit_qualification & 16' check we have
is hard to read so 'string' here works as a comment :-)
I would've liked the patch in the following shape more:
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e35552055c07..65b6062ad7bc 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4898,15 +4898,15 @@ static int handle_triple_fault(struct kvm_vcpu *vcpu)
static int handle_io(struct kvm_vcpu *vcpu)
{
unsigned long exit_qualification;
- int size, in, string;
+ int size, in;
unsigned port;
exit_qualification = vmx_get_exit_qual(vcpu);
- string = (exit_qualification & 16) != 0;
++vcpu->stat.io_exits;
- if (string)
+ /* String instruction */
+ if (exit_qualification & BIT(4))
return kvm_emulate_instruction(vcpu, 0);
port = exit_qualification >> 16;
Also, the changelog needs to be re-phrased to state code cleanup and not
CPU cycles optimization.
--
Vitaly
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles.
2020-12-10 8:15 [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles Stephen Zhang
2020-12-10 9:56 ` Vitaly Kuznetsov
@ 2020-12-12 0:34 ` Paolo Bonzini
1 sibling, 0 replies; 3+ messages in thread
From: Paolo Bonzini @ 2020-12-12 0:34 UTC (permalink / raw)
To: Stephen Zhang, seanjc, vkuznets, wanpengli, jmattson, joro, tglx,
mingo, bp, x86, hpa
Cc: kvm, linux-kernel
On 10/12/20 09:15, Stephen Zhang wrote:
> code changes in handle_io() to save some CPU cycles.
>
> Signed-off-by: Stephen Zhang <starzhangzsd@gmail.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 47b8357..109bcf64 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4899,15 +4899,14 @@ static int handle_triple_fault(struct kvm_vcpu *vcpu)
> static int handle_io(struct kvm_vcpu *vcpu)
> {
> unsigned long exit_qualification;
> - int size, in, string;
> + int size, in;
> unsigned port;
>
> exit_qualification = vmx_get_exit_qual(vcpu);
> - string = (exit_qualification & 16) != 0;
>
> ++vcpu->stat.io_exits;
>
> - if (string)
> + if (exit_qualification & 16)
> return kvm_emulate_instruction(vcpu, 0);
>
> port = exit_qualification >> 16;
>
I would be very surprised if there's any change in the assembly code
after this patch.
Paolo
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-12-10 8:15 [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles Stephen Zhang
2020-12-10 9:56 ` Vitaly Kuznetsov
2020-12-12 0:34 ` Paolo Bonzini
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