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

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Date:	Thu, 20 Dec 2012 08:56:17 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Dongxiao Xu" <dongxiao.xu@...el.com>
Cc:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [Xen-devel] [PATCH] xen/swiotlb: Exchange to contiguous
 memory for map_sg hook

>>> On 20.12.12 at 02:23, "Xu, Dongxiao" <dongxiao.xu@...el.com> wrote:
> Sorry, maybe I am still not describing this issue clearly.

No, at least I understood you the way you re-describe below.

> 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.

Jan

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