Re: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm

[David Laight]

On Wed, 11 Feb 2026 13:55:35 +0000
Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

> On 11/02/2026 1:43 pm, David Laight wrote:
> > On Wed, 11 Feb 2026 10:57:31 +0000
> > Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> >  
> >>> Remove explicit branch hint prefixes (.byte 0x2e / 0x3e) from VMX
> >>> inline assembly sequences.
> >>>
> >>> These prefixes (CS/DS segment overrides used as branch hints on
> >>> very old x86 CPUs) have been ignored by modern processors for a
> >>> long time. Keeping them provides no measurable benefit and only
> >>> enlarges the generated code.    
> >> It's actually worse than this.
> >>
> >> The branch-taken hint has new meaning in Lion Cove cores and later,
> >> along with a warning saying "performance penalty for misuse".
> >>
> >> i.e. "only insert this prefix after profiling".  
> > Don't they really have much the same meaning as before?  
> 
> Architecturally yes, microarchitecturally very much not.
> 
> For a branch known to the predictor, there is no effect.  If a branch
> unknown to the predictor gets decoded, it triggers a frontend flush and
> resteer.

That'll be 'decoded taken'.
I suspect that it is less 'painful' than a normal mispredict since it happens
as lot earlier.
Of course, if you get it wrong, there will be a mispredict penalty as well.

	David

> It is only useful for programs large enough to exceed the working set of
> the conditional predictor, and for which certain branches are known to
> be ~always taken.
> 
> Putting the prefix on a branch that isn't ~always taken is worse than
> not having the prefix in the first place, hence the warning.
> 
> ~Andrew
>