grant_opts is overoptimizing for space packing in a hypercall that
doesn't warrant the effort. Tweak the ABI without breaking it in order
to remove the bitfield by extending it to 8 bits.

Xen only supports little-endian systems, so the transformation from
uint32_t to uint8_t followed by 3 octets worth of padding is not an ABI
breakage.

No functional change

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
---
 xen/include/public/domctl.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 353f831e402e..b3c8271e66ba 100644
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -90,11 +90,18 @@ struct xen_domctl_createdomain {
     int32_t max_grant_frames;
     int32_t max_maptrack_frames;
 
-/* Grant version, use low 4 bits. */
-#define XEN_DOMCTL_GRANT_version_mask    0xf
-#define XEN_DOMCTL_GRANT_version(v)      ((v) & XEN_DOMCTL_GRANT_version_mask)
+    /*
+     * Maximum grant table version the domain can be configured with.
+     *
+     * Domains always start with v1 (if CONFIG_GRANT_TABLE) and can be bumped
+     * to use up to `max_grant_version` via GNTTABOP_set_version.
+     *
+     * Must be zero iff !CONFIG_GRANT_TABLE.
+     */
+    uint8_t max_grant_version;
 
-    uint32_t grant_opts;
+    /* Unused */
+    uint8_t rsvd0[3];
 
 /*
  * Enable altp2m mixed mode.
-- 
2.47.0

On 29.10.2024 19:16, Alejandro Vallejo wrote:
> grant_opts is overoptimizing for space packing in a hypercall that
> doesn't warrant the effort. Tweak the ABI without breaking it in order
> to remove the bitfield by extending it to 8 bits.
> 
> Xen only supports little-endian systems, so the transformation from
> uint32_t to uint8_t followed by 3 octets worth of padding is not an ABI
> breakage.
> 
> No functional change
> 
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
> ---
>  xen/include/public/domctl.h | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

This isn't a complete patch, is it? I expect it'll break the build without
users of the field also adjusted.

> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -90,11 +90,18 @@ struct xen_domctl_createdomain {
>      int32_t max_grant_frames;
>      int32_t max_maptrack_frames;
>  
> -/* Grant version, use low 4 bits. */
> -#define XEN_DOMCTL_GRANT_version_mask    0xf
> -#define XEN_DOMCTL_GRANT_version(v)      ((v) & XEN_DOMCTL_GRANT_version_mask)
> +    /*
> +     * Maximum grant table version the domain can be configured with.
> +     *
> +     * Domains always start with v1 (if CONFIG_GRANT_TABLE) and can be bumped
> +     * to use up to `max_grant_version` via GNTTABOP_set_version.
> +     *
> +     * Must be zero iff !CONFIG_GRANT_TABLE.
> +     */
> +    uint8_t max_grant_version;
>  
> -    uint32_t grant_opts;
> +    /* Unused */
> +    uint8_t rsvd0[3];
>  
>  /*
>   * Enable altp2m mixed mode.

Just to mention it: I think while binary compatible, this is still on the edge
of needing an interface version bump. We may get away without as users of the
removed identifiers will still notice by way of observing build failures.

Jan

Hi,

On Wed Oct 30, 2024 at 9:08 AM GMT, Jan Beulich wrote:
> On 29.10.2024 19:16, Alejandro Vallejo wrote:
> > grant_opts is overoptimizing for space packing in a hypercall that
> > doesn't warrant the effort. Tweak the ABI without breaking it in order
> > to remove the bitfield by extending it to 8 bits.
> > 
> > Xen only supports little-endian systems, so the transformation from
> > uint32_t to uint8_t followed by 3 octets worth of padding is not an ABI
> > breakage.
> > 
> > No functional change
> > 
> > Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
> > ---
> >  xen/include/public/domctl.h | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
>
> This isn't a complete patch, is it? I expect it'll break the build without
> users of the field also adjusted.

Indeed. The non-RFC version would have everything folded in one. I just wanted
to avoid Cc-ing everyone in MAINTAINERS for the same single RFC patch. It's
split by (rough) maintained area.

>
> > --- a/xen/include/public/domctl.h
> > +++ b/xen/include/public/domctl.h
> > @@ -90,11 +90,18 @@ struct xen_domctl_createdomain {
> >      int32_t max_grant_frames;
> >      int32_t max_maptrack_frames;
> >  
> > -/* Grant version, use low 4 bits. */
> > -#define XEN_DOMCTL_GRANT_version_mask    0xf
> > -#define XEN_DOMCTL_GRANT_version(v)      ((v) & XEN_DOMCTL_GRANT_version_mask)
> > +    /*
> > +     * Maximum grant table version the domain can be configured with.
> > +     *
> > +     * Domains always start with v1 (if CONFIG_GRANT_TABLE) and can be bumped
> > +     * to use up to `max_grant_version` via GNTTABOP_set_version.
> > +     *
> > +     * Must be zero iff !CONFIG_GRANT_TABLE.
> > +     */
> > +    uint8_t max_grant_version;
> >  
> > -    uint32_t grant_opts;
> > +    /* Unused */
> > +    uint8_t rsvd0[3];
> >  
> >  /*
> >   * Enable altp2m mixed mode.
>
> Just to mention it: I think while binary compatible, this is still on the edge
> of needing an interface version bump. We may get away without as users of the
> removed identifiers will still notice by way of observing build failures.
>
> Jan

If users are forced to rebuild either way, might as well prevent existing
binaries from breaking. There ought to be a strict distinction between ABI and
API compatibility because, while they typically move in lockstep, they don't
always (and this is one such an example).

Regardless, this is a discussion for the final patch if we get there.

Cheers,
Alejandro