Re: [Xen-devel] Xen-4.5 HVMOP ABI issues

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.11.14 at 14:55, <andrew.cooper3@xxxxxxxxxx> wrote:
> The problem is with continuations which reuse the upper bits of the
> input register, not with this HVMOP_op_mask specifically; the
> HVMOP_op_mask simply adds to an existing problem.  This is something
> which needs considering urgently because, as you identify above, we have
> already suffered an accidental ABI breakage with the mem-op widening.

Since we can't retroactively fix the mem-op widening, I still don't see
what's urgent here: As long as we don't change any of these masks,
nothing bad is going to happen. Of course one thing to consider with
this aspect in mind is whether to change the hvm-op or gnttab-op
masks again _before_ 4.5 goes out, based on whether we feel they're
wide enough for the (un)foreseeable future.

> 32bit HVM guests reliably form a continuation on every single iteration
> of a continuable hypercall (e.g. decrease reservation, which is the base
> of my XSA-111 PoC), making it trivial to construct a migrateable HVM
> domain which exposes the issue.

Hmm, I can't offhand see why that would be, but what you write
reads like you determined the reason? I ask because an unavoidable
use of continuations is certainly nice for making sure those code paths
get tested, but would be quite desirable to get eliminated at least for
non-debug builds.

Jan


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