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

To:	Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
Subject:	[Xen-devel] Re: Need help with fixing the Xen waitqueue feature
From:	Olaf Hering <olaf@xxxxxxxxx>
Date:	Wed, 9 Nov 2011 23:11:49 +0100
Cc:	keir.xen@xxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx
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On Wed, Nov 09, Andres Lagar-Cavilla wrote:

> After a bit of thinking, things are far more complicated. I don't think
> this is a "race." If the pager removed a page that later gets scheduled by
> the guest OS for IO, qemu will want to foreign-map that. With the
> hypervisor returning ENOENT, the foreign map will fail, and there goes
> qemu.

The tools are supposed to catch ENOENT and try again.
linux_privcmd_map_foreign_bulk() does that. linux_gnttab_grant_map()
appears to do that as well. What code path uses qemu that leads to a
crash?

> I guess qemu/migrate/libxc could retry until the pager is done and the
> mapping succeeds. It will be delicate. It won't work for pv backends. It
> will flood the mem_event ring.

There will no flood, only one request is sent per gfn in
p2m_mem_paging_populate().

Olaf

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