Re: [Xen-devel] [PATCH] x86: fix paging_log_dirty_op to work with paging guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.12.18 at 11:03, <roger.pau@xxxxxxxxxx> wrote:
> I expect the interdomain locking as a result of using a paging caller
> domain is going to be restricted to the p2m lock of the caller domain,
> as a result of the usage of copy to/from helpers.
> 
> Maybe the less intrusive change would be to just allow locking the
> caller p2m lock (that only lock) regardless of the subject domain lock
> level?

With caller != subject, and with the lock level then being higher
than all "normal" ones, this might be an option. But from the
very beginning we should keep the transitive aspect here in
mind: If Dom0 controls a PVH domain which controls a HVM one,
the Dom0 p2m lock may also need allowing to nest inside the PVH
DomU's one, so it'll be a total of two extra new lock levels even
if we restrict this to the p2m locks.

Furthermore, if we limited this to the p2m locks, it would become
illegal to obtain any of the higher lock level locks (i.e. ones that
nest inside the p2m lock) for the caller domain. I'm not sure
that's a good idea. As mentioned before, if caller locks collectively
all get obtained inside any subject domain ones, applying a bias
to all lock levels would perhaps be preferable. The question is
whether this criteria holds.

I also as of yet can't see how this could be expressed
transparently to existing code. Yet obviously having to change
various p2m_lock() invocations would be rather undesirable.

Jan



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