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

From: David Laight <[email-masked]>
Date: Wed, 25 Mar 2026 22:40:18 +0000

On Wed, 25 Mar 2026 21:37:59 +0100
Borislav Petkov <[email-masked]> wrote:

> On Tue, Mar 24, 2026 at 03:13:08PM -0700, Pawan Gupta wrote:
...
> > 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.

The BPF code that calls it doesn't use the C ABI - it just puts
a call instruction in the code it generates.
Hence all registers must be preserved.

	David

> Thx.