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Re: [Xen-devel] pvSCSI question - can a gref describe more than one page

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] pvSCSI question - can a gref describe more than one page?
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Thu, 25 Mar 2010 11:55:12 +0200
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On Thu, Mar 11, 2010 at 09:17:55AM +1100, James Harper wrote:
> > 
> > On 03/09/2010 07:18 PM, James Harper wrote:
> > > Porting the pvSCSI backend driver to the pvops kernel is turning out
> to
> > > be a bit harder than first thought, mostly because all the Linux
> > > routines that might be useful are written with the assumption that
> they
> > > would be passed a mapped buffer and they would then build an sg list
> out
> > > of that. pvSCSI already has an sg list, and all the routines that it
> > > makes use of have been deprecated since 2.6.18 (all the comments on
> the
> > > patches that remove the routines are like "nobody uses this
> anymore".
> > > D'oh.)
> > >
> > > The big complication appears to be that pvSCSI seems to think that
> an
> > > individual sg element can span a page, and so has to muck around
> > > splitting it into page-sized chunks. My assumption is that this is
> > > because the code in pvSCSI was cut&pasted from somewhere else.
> > >
> > 
> > In general its a bug if an sg entry crosses a page boundary.  We
> define
> > BIOVEC_PHYS_MERGEABLE so that we avoid doing any cross-page merges
> > (unless, I think, the underlying mfns are actually contiguous).  Does
> > that help?
> > 
> So a single gref can only describe a single page right? If that
> assumption is correct then I think I'm good.
> The code compiles now and doesn't crash Dom0 or anything, but my windows
> pvSCSI driver crashes because the gref is still in use when it gets it
> back... so still some bugs to fix.

Btw it seems Novell has pvscsi backend/frontend in their SLES11 SP1 2.6.32 
kernel tree..
if you want to take a look for the pvops port.

-- Pasi

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