Re: [Xen-devel] [PATCH] xen: fix initialization of wallclock time for PVHVM on migration

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-06-11 at 13:12 +0100, Jan Beulich wrote:
> >>> On 11.06.13 at 14:01, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> > On 06/11/2013 12:59 PM, Jan Beulich wrote:
> >>>>> On 11.06.13 at 12:46, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
> >>> The initial values of the wallclock time in the shared info page are
> >>> set for PVHVM guests when the hypercall page is initialized, since the
> >>> hypercall page is not reinitialized on resume, the hypervisor
> >>> wallclock time is not properly set on resume.
> >>>
> >>> Fix it by forcing an update of the wallclock values when the shared
> >>> info page is mapped.
> >>
> >> NACK - this is a guest side bug. After migration, a guest _has_ to
> >> re-init the hypercall page, as it may have got migrated between
> >> a VMX and an SVM machine, and the hypercall instructions are
> >> different between them.
> > 
> > Does migrating from VMX to SVM actually work?
> 
> If you sufficiently feature-restrict both hosts, it should work. At
> least it was made work years ago iirc (see the sysenter/syscall
> emulation code in the x86 emulator - the only case where this
> may be needed is when doing cross vendor migration).

Doesn't the presence of that emulation contradict the requirement to
reinit the hypercall page?

My recollection is that although it could be made to work the
performance would suffer horribly precisely due to this emulation.

Ian.



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