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RE: [Xen-devel] Assigning contiguous memory to a driver domain

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Konrad Wilk <konrad.wilk@xxxxxxxxxx>
Subject: RE: [Xen-devel] Assigning contiguous memory to a driver domain
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 20 Sep 2010 16:42:09 -0700 (PDT)
Cc: Rafal, xen-devel@xxxxxxxxxxxxxxxxxxx, Jan Beulich <JBeulich@xxxxxxxxxx>, Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>, Wojtczuk <rafal@xxxxxxxxxxxxxxxxxxxxxx>
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>  On 09/20/2010 02:41 PM, Konrad Rzeszutek Wilk wrote:
> > On Mon, Sep 20, 2010 at 01:27:24PM -0700, Jeremy Fitzhardinge wrote:
> >>  On 09/20/2010 12:48 PM, Konrad Rzeszutek Wilk wrote:
> >>> Let me expand this. During bootup Xen-SWIOTLB  (which for DomU you
> have
> >>> to enable via the 'iommu=soft'), allocated 32 2MB chunks of
> contingous
> >>> memory under the 4GB limit. Those chunks stay in DomU and are used
> >>> during the the runtime of the DomU. They don't go back to Xen
> unless the
> >>> domain has been terminated. Any of the DMA operations that any
> driver
> >>> does go through the SWIOTLB bufer if the physical (mfn) for the DMA
> is
> >>> outside the driver capabilities (say, your ping buffer is allocated
> above
> >>> the 4GB, and your r8169 can only do 32-bit, then SWIOTLB would be
> utilized
> >>> to "bounce" the memory).
> >> BTW, are there any hooks to make sure these pages are still
> contiguous
> >> after migration/restore?
> > <shudders>I don't really know. We do save the P2M map, so those
> mappings
> > are saved in the guest. But for the MFNs that have been exchanged - I
> don't
> > believe the hypervior is notified about which MFNs have been
> transfered
> > to another domain? Or these MFNs extracted during save/resume
> On save the toolstack will rewrite all the mfns in the pagetables and
> the p2m map to canonical pfn form, then rewrite them back to mfns on
> resume when it has allocated pages for them all.  I don't believe
> there's any mechanism to remember what regions were mfn-contiguous and
> restore that on resume.

I may be too far behind on this thread, but aren't we only talking about
driver domains with passthrough?   Is it even possible to migrate
driver domains yet?

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