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

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Roger Pau Monné writes ("Re: [PATCH v2 for-4.15] x86/msr: introduce an option 
for HVM relaxed rdmsr behavior"):
> On Thu, Mar 04, 2021 at 02:59:38PM +0000, Ian Jackson wrote:
> > 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.

The guest could be stopped with xl shutdown and then recrated with xl
create, from the config file.  I don't think we want to break that use
case here either.

Ian.