Re: [RFC PATCH net-next v6 12/15] tcp: RX path for devmem TCP

[Mina Almasry]

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