[Xen-devel] Re: Need help with fixing the Xen waitqueue feature

[Olaf Hering <olaf@xxxxxxxxx>]

On Tue, Nov 08, Andres Lagar-Cavilla wrote:

> Tbh, for paging to be effective, we need to be prepared to yield on every
> p2m lookup.

Yes, if a gfn is missing the vcpu should go to sleep rather than
returning -ENOENT to the caller. Only the query part of gfn_to_mfn
should return the p2m paging types.

> Let's compare paging to PoD. They're essentially the same thing: pages
> disappear, and get allocated on the fly when you need them. PoD is a
> highly optimized in-hypervisor optimization that does not need a
> user-space helper -- but the pager could do PoD easily and remove all that
> p2m-pod.c code from the hypervisor.

Perhaps PoD and paging could be merged, I havent had time to study the
PoD code.

> PoD only introduces extraneous side-effects when there is a complete
> absence of memory to allocate pages. The same cannot be said of paging, to
> put it mildly. It returns EINVAL all over the place. Right now, qemu can
> be crashed in a blink by paging out the right gfn.

I have seen qemu crashes when using emulated storage, but havent
debugged them yet. I suspect they were caused by a race between nominate
and evict.

Olaf

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