Re: [PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Mar 04, 2021 at 02:59:38PM +0000, Ian Jackson wrote:
> Roger Pau Monne writes ("[PATCH v2 for-4.15] x86/msr: introduce an option for HVM relaxed rdmsr behavior"):
> > Introduce an option to allow selecting a less strict behaviour for
> > rdmsr accesses targeting a MSR not explicitly handled by Xen. Since
> > commit 84e848fd7a162f669 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.
> ...
> > I wonder whether we need to to enable this option by default for
> > guests being migrated from previous Xen versions? Maybe that's not
> > required as the option is helpful mostly for early boot I would
> > assume, afterwards an OS should already have the #GP handler setup
> > when accessing MSRs.
> 
> I think it's almost as bad to have guests which can be migrated in,
> but which then cannot reboot.

Ups, yes, right.

> Historically we have taken the view that new Xen must support old
> guests, even if that means being bug-compatible.  So I am strongly in
> favour of avoiding such a usability regression.

I'm not a xl/libxl expert, but couldn't we set the option in a
persistent way for migrated-in guests?

IIRC at domain creation libxl knows whether it's a restore or a fresh
domain, and hence we could set the option there?

The part I'm not sure is about how to make it persistent.

Thanks, Roger.