linux-kernel - Re: [PATCH net-next 09/10] nvme-tcp: use crc32c() and skb_copy_and_crc32c_datagram

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Message-ID: <20250517174552.GB1239@sol>
Date: Sat, 17 May 2025 10:45:52 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: netdev@...r.kernel.org, linux-nvme@...ts.infradead.org,
	linux-sctp@...r.kernel.org, linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Sagi Grimberg <sagi@...mberg.me>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH net-next 09/10] nvme-tcp: use crc32c() and
 skb_copy_and_crc32c_datagram_iter()

On Thu, May 15, 2025 at 11:06:56PM -0700, Christoph Hellwig wrote:
> On Thu, May 15, 2025 at 10:31:00PM -0700, Eric Biggers wrote:
> > +static inline __le32 nvme_tcp_hdgst(const void *pdu, size_t len)
> > +{
> > +	return cpu_to_le32(~crc32c(NVME_TCP_CRC_SEED, pdu, len));
> >  }
> 
> This drops the unaligned handling.  Now in the NVMe protocol it will
> always be properly aligned, but my TCP-foo is not good enough to
> remember if the networking code will also guarantee 32-bit alignment
> for the start of the packet?
> 
> Otherwise this looks great:
> 
> Reviewed-by: Christoph Hellwig <hch@....de>

The nvme-tcp driver already assumes that the header is at least 4-byte aligned,
considering that it accesses hdr->plen and the struct doesn't use __packed:

    struct nvme_tcp_hdr {
            __u8	type;
            __u8	flags;
            __u8	hlen;
            __u8	pdo;
            __le32	plen;
    };

On the send size, the header size is always sizeof(struct nvme_tcp_cmd_pdu) or
sizeof(struct nvme_tcp_data_pdu) which are multiples of 4.  On the receive side,
nvme-tcp validates hdr->hlen == sizeof(struct nvme_tcp_rsp_pdu) and then does:

    recv_digest = *(__le32 *)(pdu + hdr->hlen);

So, using put_unaligned_le32() is unnecessary.  I just had it there in v1
because I had directly translated crypto_ahash_digest(), which does ultimately
do a put_unaligned_le32() once you unravel all the API layers.

- Eric