Re: [PATCH v8 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

Borislav Petkov
2026-03-25T20:38:55Z

On Tue, Mar 24, 2026 at 03:13:08PM -0700, Pawan Gupta wrote:
> This is cleaner. A few things to consider are, CLEAR_BRANCH_HISTORY that
> calls clear_bhb_loop() would be calling into C code very early during the
> kernel entry. The code generated here may vary based on the compiler. Any
> indirect branch here would be security risk. This needs to be noinstr so
> that it can't be hijacked by probes and ftraces.
>
> At kernel entry, calling into C before mitigations are applied is risky.

You can write the above function in asm if you prefer - should still be
easier.

> Although call to clear_bhb_loop() will be inserted at the end of the BPF
> program before it returns, I am not sure if it is safe to assume that
> trashing registers in the path clear_bhb_loop() -> __clear_bhb_loop() is
> okay? Especially, when we don't know what code compiler generated for
> clear_bhb_loop(). BPF experts would know better?

The compiler would preserve the regs. If you write it in asm and you adhere to
the C ABI, you could preserve them too. Shouldn't be too many.

Thx.

--
Regards/Gruss,
Boris.