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Re: [Xen-devel] [PATCH v2 09/20] xen/biomerge: Don't allow biovec to be merge when Linux is not using 4KB page

Hi Konrad,

On 10/07/2015 21:12, Konrad Rzeszutek Wilk wrote:
On Thu, Jul 09, 2015 at 09:42:21PM +0100, Julien Grall wrote:
When Linux is using 64K page granularity, every page will be slipt in
multiple non-contiguous 4K MFN (page granularity of Xen).

But you don't care about that on the Linux layer I think?

In general use case (i.e arch agnostic) we care about it. We don't want to merge 2 biovec if they are not living on the same MFNs.

As in, is there an SWIOTLB that does PFN to MFN and vice-versa

I thought that ARM guests are not exposed to the MFN<->PFN logic
and trying to figure that out to not screw up the DMA engine
on a PCIe device slurping up contingous MFNs which don't map
to contingous PFNs?

I will let these 2 questions for Stefano. He knows better than me swiotlb for ARM.

So far, I skipped swiotlb implementation for 64KB page granularity as I'm not sure what to do when a page is split across multiple MFNs.

Although I don't think this can happen with this specific series as:
- The memory is a direct mapping so any Linux page is using contiguous MFNs. - Foreign mapping is using the 4KB of the Linux page. This is for an easier implementation.

For the latter, I plan to work on using the Linux page to map multiple foreign gfn. I have to talk with Stefano about it how to handle it.


Julien Grall

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