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

Dave Hansen
2025-11-21T18:42:25Z

On 11/21/25 10:16, Pawan Gupta wrote:
> On Fri, Nov 21, 2025 at 08:50:17AM -0800, Dave Hansen wrote:
>> On 11/21/25 08:45, Nikolay Borisov wrote:
>>> OTOH: the global variable approach seems saner as in the macro you'd
>>> have direct reference to them and so it will be more obvious how things
>>> are setup.
>>
>> Oh, yeah, duh. You don't need to pass the variables in registers. They
>> could just be read directly.
>
> IIUC, global variables would introduce extra memory loads that may slow
> things down. I will try to measure their impact. I think those global
> variables should be in the .entry.text section to play well with PTI.

Really? I didn't look exhaustively, but CLEAR_BRANCH_HISTORY seems to
get called pretty close to where the assembly jumps into C. Long after
we're running on the kernel CR3.

> 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.

So I think we have 4-ish options at this point:

1. Generate the long and short sequences independently and in their
entirety and ALTERNATIVE between them (the original patch)
2. Store the inner/outer loop counts in registers and:
2a. Load those registers from variables
2b. Load them from ALTERNATIVES
3. Store the inner/outer loop counts in variables in memory