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Re: [Xen-devel] [PATCH 00/10] x86/hvm: pkeys, add memory protection-key support

On 17/11/15 10:26, Jan Beulich wrote:
>>>> On 16.11.15 at 18:45, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Furthermore, it is unclear (given the unwritten ABI) whether it is even
>> safe to move _PAGE_GNTTAB out of the way, as this is visible to a PV guest.
> It seems pretty clear to me that this would be unsafe: It being
> part of L1_DISALLOW_MASK, if it moved and a guest used the
> bit for its own purposes, the guest would break. I guess we'll
> need an ELF note by which the guest can advertise which of the
> available bits it doesn't care about itself.

Well - it depends whether any of these bits actually get used.

If none actually do get used, we would be better to retro-fit a real ABI
in place, without requiring all new guests to opt-in to get new features.


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