Re: [kvm-unit-tests PATCH] x86: apic, vmexit: replace nop with serialize to wait for deadline timer

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 03, 2026, Isaku Yamahata wrote:
> On Tue, Feb 03, 2026 at 10:30:06AM -0800,
> Sean Christopherson <seanjc@google.com> wrote:
> 
> > > +static inline void serialize(void)
> > > +{
> > > +	/* serialize instruction. It needs binutils >= 2.35. */
> > 
> > And a CPU that supports it...  I don't see any point in using SERIALIZE.  To check
> > for support, this code would need to do CPUID to query X86_FEATURE_SERIALIZE, and
> > CPUID itself is serializing (the big reason to favor SERIALIZE over CPUID is to
> > avoid a VM-Exit for performance reasons).
> 
> Thank you for pointing it out. I'll replace it with raw_cpuid(0, 0).
> Or do you want to opencode cpuid() in each places?

It probably makes sense to add a helper to arm the deadline timer, and deal with
the serialization there.  E.g. start_tsc_deadline_timer() also has a nop() of
dubious value.