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:	Wed, 15 Jul 2015 10:56:50 +0200
From:	Julien Grall <julien.grall@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<ian.campbell@...rix.com>, <stefano.stabellini@...citrix.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 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
> 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?


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.

Regards,

-- 
Julien Grall
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