Re: [PATCH v2 3/5] x86: don't override INVALID_M2P_ENTRY with SHARED_M2P_ENTRY

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.08.2020 15:00, Andrew Cooper wrote:
> On 24/08/2020 13:34, Jan Beulich wrote:
>> While in most cases code ahead of the invocation of set_gpfn_from_mfn()
>> deals with shared pages, at least in set_typed_p2m_entry() I can't spot
>> such handling (it's entirely possible there's code missing there). Let's
>> try to play safe and add an extra check.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I agree that this is an improvement.
> 
> Therefore, tentatively Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks, but - what do I do with a tentative ack? Take it as a "normal"
one, or not take it at all?

> However, I don't think it is legitimate for set_gpfn_from_mfn() to be
> overriding anything.
> 
> IMO, we should be asserting something like (pfn == SHARED_M2P_ENTRY) ==
> (d == dom_cow).
> 
> Any code not passing this properly is almost certainly broken anyway,
> and fixing up behind its back like this doesn't feel like a clever idea
> (in debug builds at least).

As said on v1: I agree in principle, but I'd like such a change to be
made by the mem-sharing maintainer(s), so we wouldn't notice fallout
only several months or years later. Tamas - would you be up for this?

Jan