Re: [Xen-devel] vMCE vs migration

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jan 23, 2012 at 11:08 AM, Jan Beulich <JBeulich@xxxxxxxx> 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?
 -George

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