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

[David Laight <David.Laight@ACULAB.COM>]

From: Jim Mattson
> Sent: 05 October 2022 23:29
> 
> On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@amazon.com> 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.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)