Re: [PATCH v4 04/11] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

Pawan Gupta
2025-11-24T19:22:01Z

On Fri, Nov 21, 2025 at 01:36:37PM -0800, Dave Hansen wrote:
> On 11/21/25 13:26, Pawan Gupta wrote:
> > On Fri, Nov 21, 2025 at 10:42:24AM -0800, Dave Hansen wrote:
> >> On 11/21/25 10:16, Pawan Gupta wrote:
> ...>>> Also I was preferring constants because load values from global
> variables
> >>> may also be subject to speculation. Although any speculation should be
> >>> corrected before an indirect branch is executed because of the LFENCE after
> >>> the sequence.
> >>
> >> I guess that's a theoretical problem, but it's not a practical one.
> >
> > Probably yes. But, load from memory would certainly be slower compared to
> > immediates.
>
> Yeah, but it's literally two bytes of data that can almost certainly be
> shoved in a cacheline that's also being read on kernel entry. I suspect
> it would be hard to show a delta between a memory load and an immediate.
>
> I'd love to see some actual data.

You were right, the perf-tool profiling and the Unixbench results show no
meaningful difference between the two approaches. I was irrationally biased
towards immediates. Making the loop count as global.