Re: [Xen-devel] vMCE vs migration

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.01.12 at 12:27, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> On Mon, 2012-01-30 at 13:47 +0000, Jan Beulich wrote:
>> >>> On 26.01.12 at 17:54, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
>> > On Tue, 2012-01-24 at 11:08 +0000, Jan Beulich wrote:
>> >> >>> On 24.01.12 at 11:29, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> 
>> >> >>> wrote:
>> >> > 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?
>> >>
>> >> 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? :-)
>> 
>> Below/attached a draft patch (compile tested only), handling save/
>> restore of the bank count, but not allowing for a config setting to
>> specify its initial value (yet).
> 
> Looks pretty good for a first blush.  Just one question: Why is the vmce
> count made on a per-vcpu basis, rather than on a per-domain basis, like
> the actual banks are?  Is the host MCE stuff per-vcpu?

The question should probably be the other way around - why is the
vMCE implementation using global (fake) MSRs rather than per-vCPU
ones (as they would be on hardware). If the change here was
implemented as per-domain MSRs, a future move of the vMCE
implementation to a more natural model would be impossible. Also,
for the PV case the save/restore logic is much simpler this way.

Jan


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