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

To: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Subject: Re: [Xen-devel] Assigning contiguous memory to a driver domain
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 20 Sep 2010 14:55:20 -0700
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Jan Beulich <JBeulich@xxxxxxxxxx>, Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>, Rafal 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.


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