Re: [Xen-devel] [PATCH 5 of 9] Fine-grained concurrency control structure for the p2m

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

I view PoD as a special case of paging, with sweeps implemented by the
user-space pager, etc.

But the question that lingers in my mind is what do you do in modes that
don't support ept+paging today (software shadow, amd npt).

Also, paging needs to get waitqueues before being a palatable replacement
for PoD, imho. We're aiming in that direction too.

Andres
> Hi,
>
> At 07:20 -0700 on 02 Nov (1320218409), andres@xxxxxxxxxxxxxxxx wrote:
>> > I suspect that if this is a contention point, allowing multiple
>> readers
>> > will become important, especially if there are particular pages that
>> > often get emulated access.
>> >
>> > And also, I'd  like to get some sort of plan for handling long-lived
>> > foreign mappings, if only to make sure that this phase-1 fix doesn't
>> > conflict wih it.
>> >
>>
>> If foreign mappings will hold a lock/ref on a p2m subrange, then they'll
>> disallow global operations, and you'll get a clash between log-dirty
>> and,
>> say, qemu. Ka-blam live migration.
>
> Yep.  That's a tricky one.  Log-dirty could be special-cased but I guess
> we'll have the same problem with paging, mem-event &c. :(
>
>> Read-only foreign mappings are only problematic insofar paging happens.
>> With proper p2m update/lookups serialization (global or fine-grained)
>> that
>> problem is gone.
>>
>> Write-able foreign mappings are trickier because of sharing and w^x. Is
>> there a reason left, today, to not type PGT_writable an hvm-domain's
>> page
>> when a foreign mapping happens?
>
> Unfortunately, yes.  The shadow pagetable code uses the typecount to
> detect whether the guest has any writeable mappings of the page; without
> that it would have to brute-force search all the L1 shadows in order to
> be sure that it had write-protected a page.
>
>> That would solve sharing problems. w^x
>> really can't be solved short of putting the vcpu on a waitqueue
>> (preferable to me), or destroying the mapping and forcing the foreign OS
>> to remap later. All a few steps ahead, I hope.
>
> OK, so if I understand correctly your plan is to add this mutual
> exclusion for all other users of the p2m (emulation &c) but leave
> foreign mappings alone for now, with the general plan of fixing that up
> using waitqueues.  That's OK by me.
>
>> Who/what's using w^x by the way? If the refcount is zero, I think I know
>> what I'll do ;)
>
> I think the original authors are using it in their product.  I haven't
> heard of any other users but there might be some.
>
>> What is a real problem is that pod sweeps can cause deadlocks. There is
>> a
>> simple step to mitigate this: start the sweep from the current gfn and
>> never wrap around -- too bad if the gfn is too high. But this alters the
>> sweeping algorithm. I'll deal with it when its it's turn.
>
> OK.  If there's some chance that Olaf can make PoD a special case of
> paging maybe we can get rid of the sweeps altogether (i.e., have the
> domain pause when it runs out of PoD and let the pager fix it up).  But
> I know George has spent a fair amount of time tuning the performance of
> PoD so that may not be acceptable.
>
> Cheers,
>
> Tim.
>



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