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

[Laurent Vivier <lvivier@redhat.com>]

On 9/12/26 12:57, Ammar Yasser wrote:
> 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 ?

I think we can keep the original value.

> 
>>
>>> +		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

I was speaking about the sizeof(thdr->vnet_len)

Thanks,
Laurent>>
>>> +
>>> +	return ret;
>>>    }
>>>    
>>>    /**
>