[Xen-devel] Re: Re: mapping problems in xenpaging

[Tim Deegan <tim@xxxxxxx>]

At 21:20 -0400 on 09 Oct (1318195224), Andres Lagar Cavilla wrote:
> I have a proposal. I'd like to hear from the list what they think.
> 
> - 1. change p2m lock to a read/write lock
> - 2. Make lookups (gfn_to_mfn_* family) take a read lock. All current
> callers of p2m_lock will become write lockers.
> - 3. Change the gfn_to_mfn_* family to get_page on the mfn obtained,
> while holding the read lock.
> - 4. Have all lookup callers put_page on the obtained mfn, once done.

This seems like a step in the right direction, but if we're going to
make this big an interface change there might be better interfaces to
end up with.

A few issues I can see with it:
 - p2m lookups are on some very performance-sensitive paths
   (e.g. multiple times in any pagetable walk or instruction emulation
   in a HVM guest) so adding the rwlock might have a noticeable impact.

 - This fixes one class of races (page gets freed-to-xen underfoot) but
   leaves another one (gfn -> mfn map changes underfoot) untouched.  In
   particular it doesn't solve the race where a foreign mapper 
   gets a r/w map of what's meant to be a read-only frame.

I think that to fix things properly we need to have some refcount
associated with the p2m mapping itself.  That would be taken by all
lookups (or at least some - we could have a flag to the p2m lookup) and
released as you suggest, but more importantly it would block all p2m changes
while the count was raised.  (I think that a least in the common case we
could encode such a refcount using the existing typecount).

One problem then is how to make all the callers of the p2m update
functions handle failure, either by waiting (new deadlock risk?) or
returning EAGAIN at the hypercall interface.  Paths where the update
isn't caused by an explicit request (like log-dirty and the mem-event
rx-to-rw conversion) would be particularly tricky.  

More seriously, it introduces another round of the sort of priority
inversion we already get with the existing refcounts - a foreign
mapping, caused by a user-space program in another VM, could arbitrarily
delay a p2m update (and so prevent a VCPU from making progress), without
any mechanism to even request that the mapping be removed.

Any ideas how to avoid that?  Potentially with some extra bookkeeping on
foreign mappings we could revoke or redirect them when the p2m changes.
That would fit nicely with the abstraction in the interfaces where HVM
domains' memory is always indexed by pfn.  I can imagine it being quite
tricky though.

> I'm more wary that turning p2m locking into read/write will result in
> code deadlocking itself: taking a read lock first and a write lock
> later. Possibly the current rwlock implementation could be improved to
> keep a cpumask of read-lockers, and provide an atomic "promote from
> read to write" atomic operation (something along the lines of wait
> until you're the only reader in the cpumask, and then cmpxchg(lock,
> -1, WRITE_BIAS))

I think that would deadlock if two cpus tried it at once.

Tim.

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