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

[Jan Beulich <jbeulich@suse.com>]

On 23.11.2020 13:26, Andrew Cooper wrote:
> 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@suse.com>
>>> ---
>>> v2: Don't change type's type.
>> Ping?
> 
> Ping what?  There is still nothing addressing my concerns from v1.

I did reply to your concerns on Sep 11th, and then replied to this
reply of mine another time on Sep 28th. Neither of these got any
response from you, hence I had to conclude - after two further
pings on v1 - that they were satisfactory to you. Now you say they
weren't, but without saying in which way, so I still wouldn't know
what to change in the description.

On the code change itself you did say "... so this is probably a
good change", so I was further understanding that your concern is
merely with the description. Maybe I misunderstood this aspect,
too?

> 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.

Until now it has been my understanding that they just don't happen
to manifest, because guests know to behave themselves (read: pin,
first and foremost, all their page tables, which means we wouldn't
in practice run into ones with an in-flight state change).

Jan