Re: [PATCH v2 13/13] vhost-user,udp: Use 2 iovec entries per element

[Laurent Vivier <lvivier@redhat.com>]

On 3/12/26 05:39, David Gibson wrote:
> On Mon, Mar 09, 2026 at 10:47:44AM +0100, Laurent Vivier wrote:
>> iPXE places the vnet header in one virtqueue descriptor and the payload
>> in another. When passt maps these descriptors, it needs two iovecs per
>> virtqueue element to handle this layout.
>>
>> Without this, passt crashes with:
>>
>>    ASSERTION FAILED in virtqueue_map_desc (virtio.c:403): num_sg < max_num_sg
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>>   udp_vu.c    |  8 ++++----
>>   vu_common.c | 15 +++++++++++----
>>   2 files changed, 15 insertions(+), 8 deletions(-)
>>
>> diff --git a/udp_vu.c b/udp_vu.c
>> index cb5274aa1d81..47659b0402fd 100644
>> --- a/udp_vu.c
>> +++ b/udp_vu.c
>> @@ -34,7 +34,7 @@
>>   #include "vu_common.h"
>>   
>>   static struct iovec     iov_vu		[VIRTQUEUE_MAX_SIZE];
>> -static struct vu_virtq_element	elem		[VIRTQUEUE_MAX_SIZE];
>> +static struct vu_virtq_element	elem	[VIRTQUEUE_MAX_SIZE / IOV_PER_ELEM];
> 
> The two level structure of the queues (array of elems, each pointing
> to an array of iovs) confuses me a bit.  I would have thought that
> VIRTQUEUE_MAX_SIZE represented the maximum number of elements, even if
> those elements had multiple iovs.  Can you enlighten me?
> 
In all the code until now I supposed the number of iovec is equal to the number of element 
because we had only one iovec per element.

At virtio level, VIRTQUEUE_MAX_SIZE is the number of vring_desc in a virtqueue, and a 
vring_desc is translated to an iovec in virtqueue_map_desc() that are collected to an 
element in vu_queue_map_desc(). So in passt, the maximum number of collected iovec will be
VIRTQUEUE_MAX_SIZE and the maximum of element is VIRTQUEUE_MAX_SIZE if we have only one 
iovec per element.

I'm reworking all of this to be able to have several iovec per element on demand (in 
vu_queue_map_desc()). This patch and previous one will be dropped.

Thanks,
Laurent