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

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Message-ID: <20150716183059.GC14309@x230.dumpdata.com>
Date:	Thu, 16 Jul 2015 14:30:59 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Julien Grall <julien.grall@...rix.com>
Cc:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	ian.campbell@...rix.com, linux-kernel@...r.kernel.org,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: 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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