Re: [PATCH v4 1/4] x86/spec-ctrl: add logic to issue IBPB on exit to guest

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Dec 18, 2023 at 02:50:27PM +0100, Jan Beulich wrote:
> On 18.12.2023 14:46, Jan Beulich wrote:
> > On 18.12.2023 13:11, Roger Pau Monné wrote:
> >> Hello,
> >>
> >> I'm not as expert as Andrew in all the speculation stuff, but I will
> >> try to provide some feedback.
> >>
> >> On Tue, Feb 14, 2023 at 05:10:42PM +0100, Jan Beulich wrote:
> >>> In order to be able to defer the context switch IBPB to the last
> >>> possible point, add logic to the exit-to-guest paths to issue the
> >>> barrier there, including the "IBPB doesn't flush the RSB/RAS"
> >>> workaround. Since alternatives, for now at least, can't nest, emit JMP
> >>> to skip past both constructs where both are needed. This may be more
> >>> efficient anyway, as the sequence of NOPs is pretty long.
> >>
> >> Could you elaborate on the reason why deferring the IBPB to the exit
> >> to guest path is helpful?  So far it just seem to make the logic more
> >> complex without nay justification (at least in the changelog).
> > 
> > I've added "(to leave behind as little as possible)" ahead of the 1st
> > comma - is that sufficient, do you think?
> 
> Actually, the next patch supplies better context, i.e. is more / also
> about avoiding to clobber Xen's own predictions.

Right, that I got from next patch, but given context switch is already
a quite heavy operation, does avoiding the cleaning of the branch
predictor make that much of a difference?

IMO it needs good justification given it's a change that makes the
logic harder to follow, so if it turns out there's no difference I
would rather leave the IBPB at context switch.

Thanks, Roger.