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

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Message-Id: <a2d926be-695a-484b-b2b5-098da47e372e@app.fastmail.com>
Date: Tue, 05 Mar 2024 09:41:55 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mina Almasry" <almasrymina@...gle.com>, Netdev <netdev@...r.kernel.org>,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc: "David S . Miller" <davem@...emloft.net>,
 "Eric Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>,
 "Paolo Abeni" <pabeni@...hat.com>, "Jonathan Corbet" <corbet@....net>,
 "Richard Henderson" <richard.henderson@...aro.org>,
 "Ivan Kokshaysky" <ink@...assic.park.msu.ru>,
 "Matt Turner" <mattst88@...il.com>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
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 "Helge Deller" <deller@....de>, "Andreas Larsson" <andreas@...sler.com>,
 "Jesper Dangaard Brouer" <hawk@...nel.org>,
 "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
 "Steven Rostedt" <rostedt@...dmis.org>,
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 "Martin KaFai Lau" <martin.lau@...ux.dev>,
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 "Yonghong Song" <yonghong.song@...ux.dev>,
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 shuah <shuah@...nel.org>, "Sumit Semwal" <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 "Pavel Begunkov" <asml.silence@...il.com>, "David Wei" <dw@...idwei.uk>,
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 "Willem de Bruijn" <willemb@...gle.com>,
 "Kaiyuan Zhang" <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH net-next v6 12/15] tcp: RX path for devmem TCP

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?

> 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?

       Arnd