Re: [PATCH 2/7] conf: Add context fields, epoll types and --vhost-kernel flag to pasta

["Ammar Yasser" <aerosound161@gmail.com>]

On Wed Sep 9, 2026 at 3:35 PM EEST, Laurent Vivier wrote:
> On 9/4/26 23:28, aerosouund wrote:
>> From: Ammar Yasser <aerosound161@gmail.com>

>> + * The only structural limit the QEMU socket protocol imposes on frames is
>> + * (2^32-1) bytes, but that would be ludicrously long in practice.  For now,
>> + * limit it somewhat arbitrarily to 65535 bytes.  FIXME: Work out an appropriate
>> + * limit with more precision.
>> + */
>> +#define L2_MAX_LEN_PASST	USHRT_MAX
>> +
>> +/** L2_MAX_LEN_VU - Maximum frame length for vhost-user mode (with L2 header)
>> + *
>> + * vhost-user allows multiple buffers per frame, each of which can be quite
>> + * large, so the inherent frame size limit is rather large.  Much larger than is
>> + * actually useful for IP.  For now limit arbitrarily to 65535 bytes. FIXME:
>> + * Work out an appropriate limit with more precision.
>> + */
>> +#define L2_MAX_LEN_VU		USHRT_MAX
>> +
>> +/* Number of descriptors in each vhost-net virtqueue */
>> +#define VHOST_NDESCS		128
>> +
>> +/* Bytes of pkt_buf backing one from-guest descriptor: a maximum size frame
>> + * plus the virtio-net header the kernel writes in front of it
>> + */
>> +#define VHOST_DESC_BYTES	(L2_MAX_LEN_PASTA +			\
>> +				 sizeof(struct virtio_net_hdr_mrg_rxbuf))
>> +
>> +/* One maximum size frame per vhost-net descriptor */bbb
>> +#define PKT_BUF_BYTES		(VHOST_NDESCS * VHOST_DESC_BYTES)
>
> 128 * (65535 + 12) = 8390016
>
> but we have in passt.c
>
> madvise(pkt_buf, sizeof(pkt_buf), MADV_HUGEPAGE);
>
> originaly we had:
>
> 2dbed699e78e ("passt: Align pkt_buf to PAGE_SIZE (start and size), try to fit in huge pages")
>
> And your change is reverting the logic of:
>
> b6945e055376 ("Simplify sizing of pkt_buf")
>

Hmm, i see the reasoning behind why it was this way before now.
Generally speaking, i think it's pretty unlikely that the there will be
a time where the packet buffer will be completely full (the kernel
queued 128 packets and i didn't process any). Only in this case would
the fact that this buffer is 8MB and doesn't have space for the
virtio_net_mrg_rxbuf cause problems. So maybe we can just put it back to
be 8MB. what do you think ?

>
>> +		ret.iov_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
>> +	} else {
>> +		ret.iov_len = c->mode == MODE_PASST ? sizeof(thdr->vnet_len) : 0;
>> +	};
>
> no need for the semicolon.
>
> This change should be in PATCH 1

This won't work unfortunately. Since this change depends on the fields
added to the context. the c->vhost.fd field
>
>> +
>> +	return ret;
>>   }
>>   
>>   /**