Re: [PATCH 2/2] KVM: VMX: Use LEAVE in vmx_do_interrupt_irqoff()

[Uros Bizjak]

On Tue, Apr 15, 2025 at 3:05 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Mon, Apr 14, 2025, Uros Bizjak wrote:
> > Micro-optimize vmx_do_interrupt_irqoff() by substituting
> > MOV %RBP,%RSP; POP %RBP instruction sequence with equivalent
> > LEAVE instruction. GCC compiler does this by default for
> > a generic tuning and for all modern processors:
>
> Out of curisoity, is LEAVE actually a performance win, or is the benefit essentially
> just the few code bytes saves?

It is hard to say for out-of-order execution cores, especially when
the stack engine is thrown to the mix (these two instructions, plus
following RET, all update %rsp).

The pragmatic solution was to do what the compiler does and use the
compiler's choice, based on the tuning below.

> > DEF_TUNE (X86_TUNE_USE_LEAVE, "use_leave",
> >         m_386 | m_CORE_ALL | m_K6_GEODE | m_AMD_MULTIPLE | m_ZHAOXIN
> >         | m_TREMONT | m_CORE_HYBRID | m_CORE_ATOM | m_GENERIC)

The tuning is updated when a new target is introduced to the compiler
and is based on various measurements by the processor manufacturer.
The above covers the majority of recent processors (plus generic
tuning), so I guess we won't fail by following the suit. OTOH, any
performance difference will be negligible.

> > The new code also saves a couple of bytes, from:
> >
> >   27: 48 89 ec                mov    %rbp,%rsp
> >   2a: 5d                      pop    %rbp
> >
> > to:
> >
> >   27: c9                      leave

Thanks,
Uros.