Re: [Xen-devel] [PATCH 4 of 5] Add support for mapping grant references into HVM guests by modifying the P2M

[Steven Smith <steven.smith@xxxxxxxxxx>]

> I understand that we are going to drop Xen 32bit, and support only Xen
> 64bit on this feature??
That's the plan, yes.

> I believe that Xen 32bit is still important and desired by many
> people. So please considere supporting that with your patch.
The problem is that we only have a limited number of bits available in
the P2M entries, which limits the number of distinct types of P2M
entry which we can encode.  On 32-bit Xen, that limit is eight, of
which seven are already used, and the grant mapping patch needs
another two (one for read-write maps, and one for read-only).  There
just isn't enough room everything we want to do.

It'd be easy enough to make grant mapping work on 32-bit Xen if we
were willing to drop some other feature (populate-on-demand, say), but
I don't think that would be an acceptable solution.  Finding somewhere
else to stash the required bits would significantly complicate things,
and, since there aren't any particularly convincing use-cases yet for
even the 64-bit variant, I didn't think that would be worthwhile.

Steven.

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