Re: [Xen-devel] [PATCH] Ignore non-zero data in unused xsave area.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.11.14 at 15:35, <dkoch@xxxxxxxxxxx> wrote:
> On Tue, 18 Nov 2014 10:26:31 -0500
> Don Koch <dkoch@xxxxxxxxxxx> wrote:
> 
>> If we restore an xsave area from an older xen that has a larger
>> size than the xcr0 bit call for, it is possible to have non-zero
>> data in the unused area if an xsave has ever been done that used
>> that area (e.g. during a context switch). Since the vcpu's xsave
>> area is never zeroed after the initial allocation, that data is
>> still there. Since we are told that said area was not written to
>> during the save or migration, there is no need to restore it.
> 
> Found the issue. Don Slutz had reported this occurring a couple of times
> and I witnessed the bad data in the unused area. Turns out he had
> done a migration from 4.3 to 4.4, then another migration from 4.4 to
> 4.3 and when he tried another 4.3 to 4.4 migration, it failed.

With the important part being that 4.3.0 (other than 4.3.3)
copied as much data as the receiving system's XSAVE
capabilities would require, i.e. potentially stuff that wasn't part
of the CPU_XSAVE save record at all (if the sending side either
had less capabilities or used an already fixed hypervisor).

Jan


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