Re: [PATCH v2] Sanity check xsave area when migrating or restoring from older Xen verions

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 20/10/14 11:21, Jan Beulich wrote:
>>>> On 18.10.14 at 01:36, <andrew.cooper3@citrix.com> wrote:
>> On 17/10/2014 18:11, Don Koch wrote:
>>> +
>>> +        /* Check to see if the xsave_area is the maximum size.
>>> +           If so, it is likely the save is from an older xen. */
>>> +        cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
>> This check is bogus for heterogeneous hardware.  We have no way of 
>> calculating what the maximum xsave area size was on the sending side 
>> should have been...
> Actually we have a way, using the xfeature_mask field that you
> made being ignored a while back. And I think applying sanity
> checks where they can be applied is a good thing. But of course
> we can't blindly compare against the full size found on the receiving
> host. We could get the size from xstate_ctxt_size() unless the
> sending host had features we don't have, in which case we'd need
> to resort to manually calculating the value.

I am not in favour of reinstating that check.

Whether the state was valid for the sending side, is not something the
receiving side should care about.

All the receiving side should care about is whether the state received
is valid.  In this case, reinstating the check still doesn't allow us to
correctly calculate the size, and manually doing so is fragile and very
prone to error.

If the record is overly long, but the trailing space is all zeroes, the
state is valid whether or not it is the correct length for the sending side.

~Andrew