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 03:20:34PM +0000, Ian Jackson wrote:
> > 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.
> 
> So my original approach was to actually risk breaking creation from
> config file and require the user to set the rdmsr_relaxed option, and
> report the problem upstream. I think ideally we would like to get to a
> point where we could drop the rdmsr_relaxed option, but maybe that's
> too optimistic.

Isn't there some way we can move in this direction without the first
thing that users experience being their guests not being able to be
created ?

Maybe we could print a warning on the console or something ?

> We have done quite a lot of testing of this new policy, but obviously
> it's not possible to test all possible guest OSes. Forcing the new
> policy by default might be too risky, so indeed falling back to
> enabling this by default could be the only solution.
> 
> The main downside of enabling by default is that then we have to
> resign to always having this kind of quirky behavior for MSR
> accesses as the default.

What would stop us changing the default later, when we had a better
idea of the set of RDMSRs that need to be special-cased ?

Ian.