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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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>

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).

~Andrew