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.
