Re: [PATCH] kvm:vmx:code changes in handle_io() to save some CPU cycles.

From: Vitaly Kuznetsov
Date: Thu Dec 10 2020 - 04:59:21 EST


Stephen Zhang <starzhangzsd@xxxxxxxxx> writes:

> code changes in handle_io() to save some CPU cycles.
>
> Signed-off-by: Stephen Zhang <starzhangzsd@xxxxxxxxx>
> ---
> 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