From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <Ian.Campbell@citrix.com>
Subject: Re: Fatal crash on xen4.2 HVM + qemu-xen dm + NFS
Date: Tue, 22 Jan 2013 10:05:16 +0000
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To: Alex Bligh <alex@alex.org.uk>
Cc: Konrad Wilk <konrad.wilk@oracle.com>, Xen Devel <xen-devel@lists.xen.org>, Jan Beulich <JBeulich@suse.com>, Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>
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On Mon, 2013-01-21 at 18:14 +0000, Alex Bligh wrote:
> Ian,
> 
> --On 21 January 2013 17:32:54 +0000 Ian Campbell <Ian.Campbell@citrix.com> 
> wrote:
> 
> > You would also need to malloc/free the buffer you are copying to/from. I
> > don't know how hard that is within qemu.
> 
> I think it's ioreq_map and ioreq_unmap within hw/xen_disk.c. I had
> foolishly assumed xc would do the grant copy, but it looks like
> as you say I need malloc/free (or mmap equivalents) + memcpy.
> 
> Is this a useful approach to try?

I've never looked inside xen_qdisk so I can't really advise, but that
sound broadly correct, except you'd want to use gnttab_copy not mmap
+memcpy. e.g. (totally making up the API because I'm too lazy to go
look):
	buffer = malloc(SIZE)
	if (writing)
		gnttab_copy_from(buffer, gntref, size) 

	io->buffer = buffer
	submit(io)
... wait for completion:
	if (reading)
		gnttab_copy_to(...)
	free(io->buffer)

Hopefully you get the idea ;-) Sounds like before the ... goes in
ioreq_map and after the ... goes in ioreq_unmap.

Ian.