Re: [PATCH v4 01/11] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()

[Nikolay Borisov]

On 11/20/25 18:56, Pawan Gupta wrote:
> On Thu, Nov 20, 2025 at 06:15:32PM +0200, Nikolay Borisov wrote:
> > On 11/20/25 08:17, Pawan Gupta wrote:
> > > Currently, BHB clearing sequence is followed by an LFENCE to prevent
> > > transient execution of subsequent indirect branches prematurely. However,
> > > LFENCE barrier could be unnecessary in certain cases. For example, when
> > > kernel is using BHI_DIS_S mitigation, and BHB clearing is only needed for
> > > userspace. In such cases, LFENCE is redundant because ring transitions
> > > would provide the necessary serialization.
> > >
> > > Below is a quick recap of BHI mitigation options:
> > >
> > >     On Alder Lake and newer
> > >
> > >     - BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
> > >                  performance overhead.
> > >     - Long loop: Alternatively, longer version of BHB clearing sequence
> > > 	       on older processors can be used to mitigate BHI. This
> > > 	       is not yet implemented in Linux.
> >
> > I find this description of the Long loop on "ALder lake and newer" somewhat
> > confusing, as you are also referring "older processors". Shouldn't the
> > longer sequence bet moved under "On older CPUs" heading? Or perhaps it must
> > be expanded to say that the long sequence could work on Alder Lake and newer
> > CPUs as well as on older cpus?
>
> Ya, it needs to be rephrased. Would dropping "on older processors" help?
>
>      - Long loop: Alternatively, longer version of BHB clearing sequence
> 		 can be used to mitigate BHI. This is not yet implemented
> 		 in Linux.

nit: Perhaps a sentence about why long loop version might be used on 
newer parts in certain cases or why it shouldn't.

> > >     On older CPUs
> > >
> > >     - Short loop: Clears BHB at kernel entry and VMexit.
>
> And also talk about "Long loop" effectiveness here:
>
>      On older CPUs
>
>      - Short loop: Clears BHB at kernel entry and VMexit. The "Long loop"
> 		  is effective on older CPUs as well, but should be avoided
> 		  because of unnecessary overhead.
> > <snip>

In any case it's much better and indeed clear!

Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>