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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Date:	Thu, 16 Jul 2015 17:15:41 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
CC:	<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

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.

Regards,

-- 
Julien Grall
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