Re: [Xen-devel] [PATCH] xen/swiotlb: Exchange to contiguous memory for map_sg hook

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, December 20, 2012 4:56 PM
> To: Xu, Dongxiao
> Cc: xen-devel@xxxxxxxxxxxxx; Konrad Rzeszutek Wilk;
> linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxx> 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.

Sorry for the late response about this thread.

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?

If we fix it in device driver side, then we may not need to modify the 
xen-swiotlb part.

Thanks,
Dongxiao

> 
> Jan


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