On Tue, Mar 5, 2024 at 12:42 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Tue, Mar 5, 2024, at 03:01, Mina Almasry wrote:
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > #define SO_PEERPIDFD 77
> > +#define SO_DEVMEM_LINEAR 79
> > +#define SO_DEVMEM_DMABUF 80
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > #define SO_PEERPIDFD 77
> > +#define SO_DEVMEM_LINEAR 79
> > +#define SO_DEVMEM_DMABUF 80
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > #define SO_PEERPIDFD 0x404B
> > +#define SO_DEVMEM_LINEAR 98
> > +#define SO_DEVMEM_DMABUF 99
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > #define SO_PEERPIDFD 0x0056
> > +#define SO_DEVMEM_LINEAR 0x0058
> > +#define SO_DEVMEM_DMABUF 0x0059
> > --- a/include/uapi/asm-generic/socket.h
> > +++ b/include/uapi/asm-generic/socket.h
> > @@ -135,6 +135,11 @@
> > #define SO_PEERPIDFD 77
> > +#define SO_DEVMEM_LINEAR 98
> > +#define SO_DEVMEM_DMABUF 99
>
> These look inconsistent. I can see how you picked the
> alpha and mips numbers, but how did you come up with
> the generic and parisc ones? Can you follow the existing
> scheme instead?
>
Sorry, yes, this is a bit weird. I'll change this to use the next
available entry rather than leave a gap.
> > diff --git a/include/uapi/linux/uio.h b/include/uapi/linux/uio.h
> > index 059b1a9147f4..ad92e37699da 100644
> > --- a/include/uapi/linux/uio.h
> > +++ b/include/uapi/linux/uio.h
> > @@ -20,6 +20,16 @@ struct iovec
> > __kernel_size_t iov_len; /* Must be size_t (1003.1g) */
> > };
> >
> > +struct dmabuf_cmsg {
> > + __u64 frag_offset; /* offset into the dmabuf where the frag starts.
> > + */
> > + __u32 frag_size; /* size of the frag. */
> > + __u32 frag_token; /* token representing this frag for
> > + * DEVMEM_DONTNEED.
> > + */
> > + __u32 dmabuf_id; /* dmabuf id this frag belongs to. */
> > +};
>
> This structure requires a special compat handler to run
> x86-32 binaries on x86-64 because of the different alignment
> requirements. Any uapi-visible structures should be defined
> to avoid this and just have no holes in them. Maybe extend
> one of the __u32 members to __u64 or add another 32-bit padding field?
>
Honestly the 32-bit fields as-is are somewhat comically large. I don't
think extending the __u32 -> __u64 is preferred because I don't see us
needing that much, so maybe I can add another 32-bit padding field.
Does this look good to you?
struct dmabuf_cmsg {
__u64 frag_offset;
__u32 frag_size;
__u32 frag_token;
__u32 dmabuf_id;
__u32 ext; /* reserved for future flags */
};
Another option is to actually compress frag_token & dmabuf_id to be
32-bit combined size if that addresses your concern. I prefer that
less in case they end up being too small for future use cases.
--
Thanks,
Mina
