Re: [Xen-devel] GNTTABOP_unmap_grant_ref doc

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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:

    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.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel