Re: [RFC PATCH 5/5] tcp, udp: Pad batched frames for vhost-user modes to 60 bytes (802.3 minimum)

[Laurent Vivier <lvivier@redhat.com>]

On 11/4/25 17:09, Stefano Brivio wrote:
> On Tue, 4 Nov 2025 16:50:43 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> On 11/3/25 11:16, Stefano Brivio wrote:
>>> For both TCP and UDP, we request vhost-user buffers that are large
>>> enough to reach ETH_ZLEN (60 bytes), so padding is just a matter of
>>> increasing the appropriate iov_len and clearing bytes in the buffer
>>> as needed.
>>>
>>> Link: https://bugs.passt.top/show_bug.cgi?id=166
>>> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
>>> ---
>>>    tcp.c          |  2 --
>>>    tcp_internal.h |  1 +
>>>    tcp_vu.c       | 27 +++++++++++++++++++++++++++
>>>    udp_vu.c       | 11 ++++++++++-
>>>    4 files changed, 38 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tcp.c b/tcp.c
>>> index e91c0cf..039688d 100644
>>> --- a/tcp.c
>>> +++ b/tcp.c
>>> @@ -335,8 +335,6 @@ enum {
>>>    };
>>>    #endif
>>>    
>>> -/* MSS rounding: see SET_MSS() */
>>> -#define MSS_DEFAULT			536
>>>    #define WINDOW_DEFAULT			14600		/* RFC 6928 */
>>>    
>>>    #define ACK_INTERVAL			10		/* ms */
>>> diff --git a/tcp_internal.h b/tcp_internal.h
>>> index 5f8fb35..d2295c9 100644
>>> --- a/tcp_internal.h
>>> +++ b/tcp_internal.h
>>> @@ -12,6 +12,7 @@
>>>    #define BUF_DISCARD_SIZE	(1 << 20)
>>>    #define DISCARD_IOV_NUM		DIV_ROUND_UP(MAX_WINDOW, BUF_DISCARD_SIZE)
>>>    
>>> +#define MSS_DEFAULT /* and minimum */	536 /* as it comes from minimum MTU */
>>>    #define MSS4				ROUND_DOWN(IP_MAX_MTU -		   \
>>>    						   sizeof(struct tcphdr) - \
>>>    						   sizeof(struct iphdr),   \
>>> diff --git a/tcp_vu.c b/tcp_vu.c
>>> index 1c81ce3..7239401 100644
>>> --- a/tcp_vu.c
>>> +++ b/tcp_vu.c
>>> @@ -60,6 +60,29 @@ static size_t tcp_vu_hdrlen(bool v6)
>>>    	return hdrlen;
>>>    }
>>>    
>>> +/**
>>> + * tcp_vu_pad() - Pad 802.3 frame to minimum length (60 bytes) if needed
>>> + * @iov:	iovec array storing 802.3 frame with TCP segment inside
>>> + * @cnt:	Number of entries in @iov
>>> + */
>>> +static void tcp_vu_pad(struct iovec *iov, size_t cnt)
>>> +{
>>> +	size_t l2len, pad;
>>> +
>>> +	ASSERT(iov_size(iov, cnt) >= sizeof(struct virtio_net_hdr_mrg_rxbuf));
>>> +	l2len = iov_size(iov, cnt) - sizeof(struct virtio_net_hdr_mrg_rxbuf);
>>> +	if (l2len >= ETH_ZLEN)
>>> +		return;
>>> +
>>> +	pad = ETH_ZLEN - l2len;
>>> +
>>> +	/* tcp_vu_sock_recv() requests at least MSS-sized vhost-user buffers */
>>> +	static_assert(ETH_ZLEN <= MSS_DEFAULT);
>>> +
>>> +	memset(&iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
>>
>> I think it should be
>>
>> 	memset((char *)iov[cnt - 1].iov_base + iov[cnt - 1].iov_len, 0, pad);
> 
> Right, thanks, I always forget that sizeof(void) being 1 is a gcc
> extension:
> 
>    https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html
> 
> What's rather confusing, actually, is that even if I explicitly enable
> -Wpointer-arith, I don't get a warning for that. Any clue?

in fact it's sizeof(void **) as iov[cnt - 1].iov_base is (void *) and you take &(void *).

Normally gcc spits out warnings when we do .iov_base + .iov_len, but as you take address 
of iov_base all is fine :P

Thanks,
Laurent