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.