Re: [PATCH v2 2/3] x86/altp2m: p2m_altp2m_get_or_propagate() should honor present page order

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Tue, Jan 4, 2022 at 4:48 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> Prior to XSA-304 the only caller merely happened to not use any further
> the order value that it passes into the function. Already then this was
> a latent issue: The function really should, in the "get" case, hand back
> the order the underlying mapping actually uses (or actually the smaller
> of the two), such that (going forward) there wouldn't be any action on
> unrelated mappings (in particular ones which did already diverge from
> the host P2M).
>
> Similarly in the "propagate" case only the smaller of the two orders
> should actually get used for creating the new entry, again to avoid
> altering mappings which did already diverge from the host P2M.

I don't really understand the reason why you want to return the
page_order from the altp2m here. The only check that uses the
page_order following is the super-page shattering check for XSA-304
but that's done on the hostp2m. So you would want to know what the
page_order is on the hosp2m, not the altp2m, no?

In either case, in all the setups we use altp2m we never use any
superpages, the recommendation is to boot with hap_1gb=0 hap_2mb=0. I
never trusted the complexity of superpage shattering and its
implementation in Xen as it is very hard to follow.

Tamas