Re: [PATCH RFC for-4.15] x86/msr: introduce an option for legacy MSR behavior selection

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

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.pau@xxxxxxxxxx>
> ---
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
> 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.

Have we diffed the result of this against 4.14 and if not would it be
a sensible thing to do ?

Ian.