linux-kernel - RE: [Xen-devel] [PATCH] xen/swiotlb: Exchange to contiguous memory for map

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Date:	Mon, 07 Jan 2013 08:46:57 +0000
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<dongxiao.xu@...el.com>
Cc:	<xen-devel@...ts.xen.org>, <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [Xen-devel] [PATCH] xen/swiotlb: Exchange to contiguous
 memory for map_sg hook

>>> "Xu, Dongxiao" <dongxiao.xu@...el.com> 01/07/13 8:17 AM >>>
> From: Jan Beulich [mailto:JBeulich@...e.com]
> >>> On 20.12.12 at 02:23, "Xu, Dongxiao" <dongxiao.xu@...el.com> wrote:
>> > Take the libata case as an example, the static DMA buffer locates
>> > (dev->link->ap->sector_buf , here we use Data Structure B in the graph) in
>> > the following structure:
>> >
>> > -------------------------------------Page boundary
>> > <Data Structure A>
>> > <Data Structure B>
>> > -------------------------------------Page boundary
>> > <Data Structure B (cross page)>
>> > <Data Structure C>
>> > -------------------------------------Page boundary
>> >
>> > Where Structure B is our DMA target.
>> >
>> > For Data Structure B, we didn't care about the simultaneous access, either
>> > lock or sync function will take care of it.
>> > What we are not sure is "read/write of A and C from other processor". As we
>> > will have memory copy for the pages, and at the same time, other CPU may
>> > access A/C.
>> 
>> The question is whether what libata does here is valid in the first
>> place - fill an SG list entry with something that crosses a page
>> boundary and is not a compound page.
>
>To make sure I understand you correctly, so do you mean the correct fix should be
> applied to libata driver, and make sure it DMA from/to correct place (for example,
> some memory allocated by DMA API), but not such certain field in a static structure?

Yes.

Jan

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