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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

On Thu, Jul 16, 2015 at 05:15:41PM +0100, Julien Grall wrote:
> Hi Stefano,
> On 16/07/2015 16:33, Stefano Stabellini wrote:
> >On Fri, 10 Jul 2015, 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?
> >>
> >>As in, is there an SWIOTLB that does PFN to MFN and vice-versa
> >>translation?
> >>
> >>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?
> >
> >Dom0 is mapped 1:1, so pfn == mfn normally, however grant maps
> >unavoidably screw up the 1:1, so the swiotlb jumps in to save the day
> >when a foreign granted page is involved in a dma operation.
> >
> >Regarding xen_biovec_phys_mergeable, we could check that all the pfn ==
> >mfn and return true in that case.
> I mentioned it in the commit message. Although, we would have to loop on
> every pfn which is slow on 64KB (16 times for every page). Given the biovec
> is called often, I don't think we can do a such things.

OK - it would be good to have the gist of this email thread in the
commit message. Thanks.
> Regards,
> -- 
> Julien Grall

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