Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR

From: pawan.kumar.gupta@xxxxxxxxxxxxxxx
Date: Thu Oct 06 2022 - 16:27:20 EST

On Thu, Oct 06, 2022 at 08:25:15AM +0000, David Laight wrote:
From: Jim Mattson
Sent: 05 October 2022 23:29

On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@xxxxxxxxxx> wrote:
> tl;dr: The existing mitigation for eIBRS PBRSB predictions uses an INT3 to
> ensure a call instruction retires before a following unbalanced RET. Replace
> this with a WRMSR serialising instruction which has a lower performance
> penalty.

The INT3 is only on a speculative path and should not impact performance.

Doesn't that depend on how quickly the cpu can abort the
decode and execution of the INT3 instruction?
INT3 is bound to generate a lot of uops and/or be microcoded.

Old cpu couldn't abort fpu instructions.
IIRC the Intel performance guide even suggested not interleaving
code and data because the data might get speculatively executed
and take a long time to abort.

I actually wonder whether 'JMPS .' (eb fe) shouldn't be used
instead of INT3 (cc) because it is fast to decode and execute.
But I'm no expect here.

I have been told that INT3 is better in this case because 'JMP .' would
waste CPU resources.