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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.06.13 at 15:59, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 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?

Definitely not - the emulation deals with system call instructions,
while the hypercall page holds hypercall instructions (which have
no emulation anywhere iirc).

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

Performance isn't going to be good, sure. It's more for some kind
of emergency I think. Wasn't it Christoph Egger who implemented
this? Maybe he recalls more details than I do...

Jan


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