Re: vMCE vs migration

[George Dunlap <george.dunlap@citrix.com>]

On Tue, 2012-01-24 at 11:08 +0000, Jan Beulich wrote:
> >>> On 24.01.12 at 11:29, George Dunlap <George.Dunlap@eu.citrix.com> wrote:
> > On Mon, Jan 23, 2012 at 11:08 AM, Jan Beulich <JBeulich@suse.com> wrote:
> >> x86's vMCE implementation lets a guest know of as many MCE reporting
> >> banks as there are in the host. While a PV guest could be expected to
> >> deal with this number changing (particularly decreasing) during migration
> >> (not currently handled anywhere afaict), for HVM guests this is certainly
> >> wrong.
> >>
> >> At least to me it isn't, however, clear how to properly handle this. The
> >> easiest would appear to be to save and restore the number of banks
> >> the guest was made believe it can access, making vmce_{rd,wr}msr()
> >> silently tolerate accesses between the host and guest values.
> > 
> > We ran into this in the XS 6.0 release as well.  I think that the
> > ideal thing to do would be to have a parameter that can be set at
> > boot, to say how many vMCE banks a guest has, defaulting to the number
> > of MCE banks on the host.  This parameter would be preserved across
> > migration.  Ideally, a pool-aware toolstack like xapi would then set
> > this value to be the value of the host in the pool with the largest
> > number of banks, allowing a guest to access all the banks on any host
> > to which it migrates.
> > 
> > What do you think?
> 
> That sounds like the way to go.

So should we put this on IanC's to-do-be-done list?  Are you going to
put it on your to-do list? :-)

 -George