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Re: [PATCH v2] x86/PV: issue branch prediction barrier when switching 64-bit guest to kernel mode
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 13 Sep 2022 18:14:37 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Tue, 13 Sep 2022 16:15:00 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Sep 13, 2022 at 06:05:30PM +0200, Jan Beulich wrote:
> 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 <jbeulich@xxxxxxxx>
> >> ---
> >> 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.
OK, didn't know there was some feedback here already. A separate
control would indeed be clearer. I guess a new patch will appear
then?
Thanks, Roger.
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