Re: [PATCH] x86/Xen: make use of IBPB controlling VM assist

[Juergen Gross <jgross@xxxxxxxx>]

On 20.03.23 14:17, Jan Beulich wrote:
> On 20.03.2023 14:02, Juergen Gross wrote:
> > On 20.03.23 11:19, Jan Beulich wrote:
> > > On 17.03.2023 14:56, Juergen Gross wrote:
> > > > +void __init xen_pv_fix_mitigations(void)
> > > > +{
> > > > +       if (!xen_vm_assist_ibpb(true))
> > > > +               setup_clear_cpu_cap(X86_FEATURE_ENTRY_IBPB);
> > > ... using both setup_clear_cpu_cap() (here) and setup_force_cpu_cap()
> > > (in retbleed_select_mitigation() won't work: The latter wins, due to
> > > how apply_forced_caps() works.
> > Oh, right.
> >
> > Just a wild guess of mine: probably the x86 maintainers would still prefer
> > a single Xen hook plus something like a setup_unforce_cpu_cap() addition.
> If so, I'm not willing to make such a patch. That's clearly more fragile
> than the approach chosen. I guess once I've made the one adjustment you
> have pointed out, I'll resubmit otherwise unchanged and include x86 folks.
> We'll see what the responses are going to be, if any at all.
Fine with me.

> > > But of course calling both functions for the same feature is bogus
> > > anyway. In fact I think it is for a good reason that in Xen we log a
> > > message in such an event.
> > Depends. For Xen we do so in the kernel for multiple features, see
> > xen_init_capabilities().
> I don't see anything there which looks like it might be both "force"d
> and "clear"ed in a single session.
Oh, I misunderstood you then.


Juergen

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