Re: Ping: [PATCH v2] x86/PV: make post-migration page state consistent

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/11/2020 12:48, Jan Beulich wrote:
> On 04.11.2020 08:56, Jan Beulich wrote:
>> When a page table page gets de-validated, its type reference count drops
>> to zero (and PGT_validated gets cleared), but its type remains intact.
>> XEN_DOMCTL_getpageframeinfo3, therefore, so far reported prior usage for
>> such pages. An intermediate write to such a page via e.g.
>> MMU_NORMAL_PT_UPDATE, however, would transition the page's type to
>> PGT_writable_page, thus altering what XEN_DOMCTL_getpageframeinfo3 would
>> return. In libxc the decision which pages to normalize / localize
>> depends solely on the type returned from the domctl. As a result without
>> further precautions the guest won't be able to tell whether such a page
>> has had its (apparent) PTE entries transitioned to the new MFNs.
>>
>> Add a check of PGT_validated, thus consistently avoiding normalization /
>> localization in the tool stack.
>>
>> Also use XEN_DOMCTL_PFINFO_NOTAB in the variable's initializer instead
>> open coding it.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> v2: Don't change type's type.
> Ping?

Ping what?  There is still nothing addressing my concerns from v1.

To re-iterate - this is a very subtle change, in a very complicated
piece of migration.  As the problems described do not manifest in
practice, it is vital to understand why.

~Andrew