Re: [RFC] x86/alternatives: Merge first and second step in text_poke_bp_batch

[Peter Zijlstra]

On Tue, Jan 14, 2025 at 03:02:37PM +0100, Jiri Olsa wrote:
> hi,
> while checking on similar code for uprobes I was wondering if we
> can merge first 2 steps of instruction update in text_poke_bp_batch
> function.
> 
> Basically the first step now would be to write int3 byte together
> with the rest of the bytes of the new instruction instead of doing
> that separately. And the second step would be to overwrite int3
> byte with first byte of the new instruction.
> 
> Would that work or do I miss some x86 detail that could lead to crash?

I *think* it will work on most modern systems, but I'm very sure I don't
have all the details.

IIRC this is the magic recipe blessed by both Intel and AMD, and
if we're going to be changing this I would want both vendors to sign off
on that.

> I tried to hack it together in attached patch and it speeds up a bit
> text_poke_bp_batch as shown below.

Why do we care about performance here?