Re: [Xen-devel] GNTTABOP_unmap_grant_ref doc

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-07-24 at 18:31 +0100, Stefano Stabellini wrote:
> On Wed, 24 Jul 2013, David Scott wrote:
> > Hi,
> > 
> > I've been working on the Mirage (mini-OS + OCaml runtime) blkback
> > implementation. It seems to be working much more happily now but I was a bit
> > surprised by the GNTTABOP_unmap_grant_ref doc -- maybe I misread it.
> > 
> > In ./xen/include/public/grant_table.h it says
> > 
> > /*
> >  * GNTTABOP_unmap_grant_ref: Destroy one or more grant-reference mappings
> >  * tracked by <handle>. If <host_addr> or <dev_bus_addr> is zero, that
> >  * field is ignored. If non-zero, they must refer to a device/host mapping
> >  * that is tracked by <handle>
> >  * NOTES:
> >  *  1. The call may fail in an undefined manner if either mapping is not
> >  *     tracked by <handle>.
> >  *  3. After executing a batch of unmaps, it is guaranteed that no stale
> >  *     mappings will remain in the device or host TLBs.
> >  */
> > 
> > When I read
> >   "If <host_addr> or <dev_bus_addr> is zero, that field is ignored"
> >   "The call may fail in an undefined manner if either mapping is not tracked
> > by <handle>"
> > 
> > I thought, "great, I'll not bother tracking anything except the handle and 
> > set
> > both fields to zero". However if I set host_addr to zero then my frontend
> > complains bitterly
> > 
> > gnttab_stubs.c: WARNING: g.e. 53 still in use! (19)
> > 
> > If I store the host_addr and fill it in during the unmap, everything seems 
> > to
> > work.
> > 
> > So did I misread the doc? Or did it mean exclusive-or i.e. you must fill in
> > either one of these two fields? Or have I done something stupid elsewhere
> > (always possible)?
> > 
> > If it turns out that I have misread the doc, I'll happily send a patch to
> > improve the text (once I'm sure I understand what it should say...)
> 
> From reading the implementation it woudl seem clear that host_addr is
> actually needed.

In fact I think you need to pass in whatever you passed to the original
mapping operation, because op->flags here is not an argument from the
unmap call but is actually a reference to the original flags used to
make the mapping. So if you made the mapping with GNTMAP_host_map then
you need to pass the host_addr on unmap. Likewise if you used
GNTMAP_device_map then you need to specify dev_bus_addr.

I've added Keir to the CC since he's the ultimate authority on these
things.

Ian.

>  In fact:
> 
>     if ( (op->host_addr != 0) && (op->flags & GNTMAP_host_map) )
>     {
>         if ( (rc = replace_grant_host_mapping(op->host_addr,
>                                               op->frame, op->new_addr, 
>                                               op->pteval,
>                                               op->flags)) < 0 )
>             goto unmap_out;
> 
>  
> no unmapping is done if host_addr is 0.
> 
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