Re: [Xen-devel] Live migration with MMIO pages

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 31/10/07 14:03, "Kieran Mansley" <kmansley@xxxxxxxxxxxxxx> wrote:

> The symptom of failure is that once live migration has started, trying
> to write to an IO page results in it getting stuck in (or a perpetual
> loop into) page_fault().  This only happens very occasionally.  I've
> interpreted it getting stuck in page_fault() as a result of the shadow
> paging which (as I understand it) marks the normal page table entries as
> read only so that writes to pages trap into the hypervisor and it can
> update its dirty set.

Yes, but then it should mark the page writable again, so that the access can
be re-executed without faulting! So this rather points at some problem with
the live-migration shadow mode w.r.t. mmio pages. You're going to have to
fix this regardless of whether you prevent xc_domain_save from mapping mmio
pages (which isn't itself a bad idea, for the reasons you note). This is
because, regardless of whether xc_domain_save ignores mmio pages, that isn't
going to change the shadow code's behaviour, and it's almost certainly that
which is causing the infinite fault loop.

 -- Keir



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