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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/02/2026 3:44 pm, Sean Christopherson wrote:
> On Wed, Feb 11, 2026, Andrew Cooper wrote:
>> On 11/02/2026 1:43 pm, David Laight wrote:
>>> On Wed, 11 Feb 2026 10:57:31 +0000
>>> Andrew Cooper <andrew.cooper3@citrix.com> 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.
>>
>> 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.
> These branches indeed ~always follow the hinted path (not taken in this case).
>
> So it sounds like this definitely isn't stable@ material, and maybe even begs
> the question if dropping the hints is a net positive?

The new behaviour only exists for hint-taken. Because it only has any
effect for a branch unknown to the predictor, the behaviour without this
hint would be as if it were a larger basic block.

hint-not-takens have no behaviour since the Pentium 4 that I'm aware of.

This change is almost certainly marginal at best.  It's not as if
VMREAD/VMWRITE lead to good code gen even at the best of times.

~Andrew