Re: [PATCH v4] tools/foreignmem: Support querying the size of a resource

[Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>]

On Thu, Jan 28, 2021 at 12:01:52PM +0000, Andrew Cooper wrote:
> With the Xen side of this interface (soon to be) fixed to return real sizes,
> userspace needs to be able to make the query.
> 
> Introduce xenforeignmemory_resource_size() for the purpose, bumping the
> library minor version.
> 
> Update both all osdep_xenforeignmemory_map_resource() implementations to
> understand size requests, skip the mmap() operation, and copy back the
> nr_frames field.
> 
> For NetBSD, also fix up the ioctl() error path to issue an unmap(), which was
> overlooked by c/s 4a64e2bb39 "libs/foreignmemory: Implement on NetBSD".
> [....]
> diff --git a/tools/libs/foreignmemory/netbsd.c 
> b/tools/libs/foreignmemory/netbsd.c
> index d26566f601..4ae60aafdd 100644
> --- a/tools/libs/foreignmemory/netbsd.c
> +++ b/tools/libs/foreignmemory/netbsd.c
> @@ -132,6 +132,10 @@ int osdep_xenforeignmemory_map_resource(
>      };
>      int rc;
>  
> +    if ( !fres->addr && !fres->nr_frames )
> +        /* Request for resource size.  Skip mmap(). */
> +        goto skip_mmap;
> +
>      fres->addr = mmap(fres->addr, fres->nr_frames << PAGE_SHIFT,
>                        fres->prot, fres->flags | MAP_ANON | MAP_SHARED, -1, 
> 0);

What happens if fres->addr is not NULL and nr_frames is 0 ?
Is it supposed to happen ? Should we assert that fres->addr is NULL when
nr_frames is 0 ? Or force fres->addr to NULL when nr_frames is 0 ?

-- 
Manuel Bouyer <bouyer@xxxxxxxxxxxxxxx>
     NetBSD: 26 ans d'experience feront toujours la difference
--