Re: [PATCH] locking/atomic/x86: Introduce try_cmpxchg64

[Peter Zijlstra]

On Tue, May 10, 2022 at 07:07:25PM +0200, Uros Bizjak wrote:
> On Tue, May 10, 2022 at 6:55 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, May 10, 2022 at 05:42:17PM +0200, Uros Bizjak wrote:
> > > This patch adds try_cmpxchg64 to improve code around cmpxchg8b.  While
> > > the resulting code improvements on x86_64 are minor (a compare and a move saved),
> > > the improvements on x86_32 are quite noticeable. The code improves from:
> >
> > What user of cmpxchg64 is this?
> 
> This is cmpxchg64 in pi_try_set_control from
> arch/x86/kvm/vmx/posted_intr.c, as shown in a RFC patch [1].

I can't read that code, my brain is hard wired to read pi as priority
inheritance/inversion.

Still, does 32bit actually support that stuff?

> There are some more opportunities for try_cmpxchg64 in KVM, namely
> fast_pf_fix_direct_spte in arch/x86/kvm/mmu/mmu.c and
> tdp_mmu_set_spte_atomic in arch/x86/kvm/mmu/tdp_mmu.c

tdp_mmu is definitely 64bit only and as such shouldn't need to use
cmpxchg64.


Anyway, your patch looks about right, but I find it *really* hard to
care about 32bit code these days.