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

[Dave Hansen]

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.

>> 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
> 
> Both of these look to be good options to me.
> 
> 2b. would be my first preference, because it keeps the loop counts as
> inline constants. The resulting sequence stays the same as it is today.
> 
>> 3. Store the inner/outer loop counts in variables in memory
> 
> I could be wrong, but this will likely have non-zero impact on performance.
> I am afraid to cause any regressions in BHI mitigation. That is why I
> preferred the least invasive approach in my previous attempts.

Your magic 8-ball and my crystal ball seem to be disagreeing today.

Time for science!