On Mon, Mar 01, 2021 at 05:16:34PM +0000, Ian Jackson wrote: > Roger Pau Monne writes ("[PATCH RFC for-4.15] x86/msr: introduce an option > for legacy MSR behavior selection"): > > Introduce an option to allow selecting the legacy behavior for > > accesses to MSRs not explicitly handled. Since commit > > 84e848fd7a162f669 and 322ec7c89f6640e accesses to MSRs not explicitly > > handled by Xen result in the injection of a #GP to the guest. This is > > a behavior change since previously a #GP was only injected if > > accessing the MSR on the real hardware will also trigger a #GP. > > > > This seems to be problematic for some guests, so introduce an option > > to fallback to this legacy behavior. The main difference between what > > was previously done is that the hardware MSR value is not leaked to > > the guests on reads. > > > > Signed-off-by: Roger Pau Monné <roger....@citrix.com> > > --- > > Cc: Boris Ostrovsky <boris.ostrov...@oracle.com> > > --- > > Note that this option is not made available to dom0. I'm not sure > > whether it makes sense to do so, since anyone updating Xen to such > > newer version will also likely pair it with a newish kernel that > > doesn't require such workarounds. > > > > RFC because there's still some debate as to how we should solve the > > MSR issue, this is one possible way, but IMO we need to make a > > decision soon-ish because of the release timeline. > > > > Boris, could you please test with Solaris to see if this fixes the > > issue? > > So AIUI this patch is to make it possible for Xen 4.15 to behave like > Xen 4.14, thus avoiding a regression for these troublesome guests.
Yes, sorry I haven't provided a release executive summary, as I wasn't sure this would be acceptable in it's current form. Can do if there's consensus this is an acceptable fix. > Have we diffed the result of this against 4.14 and if not would it be > a sensible thing to do ? I think there will likely bee too much noise, we have changed the MSR handling a bit from 4.14, so it's likely a diff to 4.14 is not going to be helpful as the context will have too many changes (albeit I haven't tried the exercise myself). Thanks, Roger.