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

[Peter Maydell]

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.

thanks
-- PMM