On 13.09.2022 17:41, Roger Pau Monné wrote: > On Tue, Jul 19, 2022 at 02:55:17PM +0200, Jan Beulich wrote: >> Since both kernel and user mode run in ring 3, they run in the same >> "predictor mode". While the kernel could take care of this itself, doing >> so would be yet another item distinguishing PV from native. Additionally >> we're in a much better position to issue the barrier command, and we can >> save a #GP (for privileged instruction emulation) this way. >> >> To allow to recover performance, introduce a new VM assist allowing the guest >> kernel to suppress this barrier. >> >> Signed-off-by: Jan Beulich <jbeul...@suse.com> >> --- >> v2: Leverage entry-IBPB. Add VM assist. Re-base. >> --- >> I'm not entirely happy with re-using opt_ibpb_ctxt_switch here (it's a >> mode switch after all, but v1 used opt_ibpb here), but it also didn't >> seem very reasonable to introduce yet another command line option. The >> only feasible alternative I would see is to check the CPUID bits directly. > > Likely needs a mention in xen-command-line.md that the `ibpb` option > also controls whether a barrier is executed by Xen in PV vCPU context > switches from user-space to kernel. The current text only mentions > vCPU context switches.
Andrew and I actually discussed this perhaps better having a separate control. Jan
