Re: [Xen-devel] [PATCH 2/4] block/xen_disk: remove use of grant map/unmap

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Apr 30, 2018 at 03:30:02PM +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Roger Pau Monne
> > Sent: 30 April 2018 16:28
> > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-block@xxxxxxxxxx; qemu-
> > devel@xxxxxxxxxx; Anthony Perard <anthony.perard@xxxxxxxxxx>; Kevin
> > Wolf <kwolf@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Max
> > Reitz <mreitz@xxxxxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH 2/4] block/xen_disk: remove use of grant
> > map/unmap
> > 
> > On Mon, Apr 30, 2018 at 04:16:52PM +0100, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Roger Pau Monne
> > > > Sent: 30 April 2018 16:12
> > > > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; qemu-block@xxxxxxxxxx; qemu-
> > > > devel@xxxxxxxxxx; Anthony Perard <anthony.perard@xxxxxxxxxx>;
> > Kevin
> > > > Wolf <kwolf@xxxxxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>;
> > Max
> > > > Reitz <mreitz@xxxxxxxxxx>
> > > > Subject: Re: [Xen-devel] [PATCH 2/4] block/xen_disk: remove use of
> > grant
> > > > map/unmap
> > > >
> > > > On Mon, Apr 30, 2018 at 01:01:37PM +0100, Paul Durrant wrote:
> > > > > The grant copy operation was added to libxengnttab in Xen 4.8.0. If
> > grant
> > > > > copy is available then data from the guest will be copied rather than
> > > > > mapped.
> > > > > The xen_disk source can be significantly simplified by removing this
> > now
> > > > > redundant code.
> > > >
> > > > Hm, I know this is a PITA, but FreeBSD gntdev hasn't implemented the
> > > > grant-copy operation yet.
> > > >
> > > > I could try to implement it, but I can't make any promises on the time
> > > > ATM, since I'm quite busy.
> > > >
> > >
> > > I guess we could carry a compat patch in QEMU that implements grant copy
> > by doing a map/memcpy/unmap , but QEMU feels like the wrong place for
> > that. I could try putting together a similar patch for the freebsd.c 
> > component
> > of libxengnttab in the xen source rather than it simply failing with ENOSYS 
> > as
> > it does now. Would either of those help?
> > 
> > Maybe this could be implemented in gnttab_core.c, so it can also be
> > used by MiniOS and Linux versions not supporting the copy ioctl as a
> > fallback?
> 
> That sounds like a reasonable idea. I'll put something together so that it 
> can go in early in 4.12.
> 

This will not work if XSM disallows grant map but allows grant copy.
Not sure how important that is.

Wei.

>   Paul
> 
> > 
> > Roger.
> 
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