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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.11.2020 12:48, Roger Pau Monné wrote:
> On Wed, Nov 04, 2020 at 08:56:50AM +0100, 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>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

> Maybe the switch could be avoided if the page is not validated or
> broken? Not that I care that much.

It certainly could be, but it didn't seem worth the code churn
to me.

Jan