Re: [RFC 06/13] rust: add bindings for memattrs

[Philippe Mathieu-Daudé]

On 6/12/24 11:59, Peter Maydell wrote:
> On Thu, 5 Dec 2024 at 18:30, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > On 12/5/24 19:15, Richard Henderson wrote:
> > > On 12/5/24 00:07, Zhao Liu wrote:
> > > > The MemTxAttrs structure is composed of bitfield members, and bindgen is
> > > > unable to generate an equivalent macro definition for
> > > > MEMTXATTRS_UNSPECIFIED.
> > >
> > > I'm happy to move away from bit fields to uint32_t or suchlike to enable
> > > MEMTXATTRS_UNSPECIFIED be a compile-time constant.
> >
> > Yeah, if we go from
> >
> > typedef struct MemTxAttrs {
> >       unsigned int unspecified:1;
> >       unsigned int secure:1;
> >       unsigned int space:2;
> >       unsigned int user:1;
> >       unsigned int memory:1;
> >       unsigned int requester_id:16;
> >       unsigned int pid:8;
> > } MemTxAttrs;
> >
> > to
> >
> > typedef struct MemTxAttrs {
> >       uint8_t unspecified;
> >       uint8_t secure;
> >       uint8_t space;
> >       uint8_t user;
> >       uint8_t memory;
> >       uint8_t pid;
> >       uint16_t requester_id;
> > } MemTxAttrs;
> >
> > is still decently packed and simplifies things a lot.
>
> The old struct is 4 bytes, and the new one is 8 bytes. We do
> a lot of directly passing 'struct MemTxAttrs' arguments around
> as arguments to functions (i.e. not passing pointers to them),
> so increasing it in size is not completely free.

Should we add a check to not pass 8 bytes?

  QEMU_BUILD_BUG_ON(sizeof(MemTxAttrs) != sizeof(uint64_t));