linux-kernel - Re: [PATCH V4 19/19] virtio

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Message-Id: <E08AB80C-D1A7-483E-9424-4F893B7161B5@linux.dev>
Date: Fri, 25 Jul 2025 13:56:15 -0700
From: Thorsten Blum <thorsten.blum@...ux.dev>
To: Jason Wang <jasowang@...hat.com>
Cc: mst@...hat.com,
 xuanzhuo@...ux.alibaba.com,
 eperezma@...hat.com,
 virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 19/19] virtio_ring: add in order support

Hi Jason,

On 23. Jul 2025, at 23:40, Jason Wang wrote:
> 
> This patch implements in order support for both split virtqueue and
> packed virtqueue. Perfomance could be gained for the device where the
> memory access could be expensive (e.g vhost-net or a real PCI device):
> 
> Benchmark with KVM guest:
> 
> Vhost-net on the host: (pktgen + XDP_DROP):
> 
>         in_order=off | in_order=on | +%
>    TX:  5.20Mpps     | 6.20Mpps    | +19%
>    RX:  3.47Mpps     | 3.61Mpps    | + 4%
> 
> Vhost-user(testpmd) on the host: (pktgen/XDP_DROP):
> 
> For split virtqueue:
> 
>         in_order=off | in_order=on | +%
>    TX:  5.60Mpps     | 5.60Mpps    | +0.0%
>    RX:  9.16Mpps     | 9.61Mpps    | +4.9%
> 
> For packed virtqueue:
> 
>         in_order=off | in_order=on | +%
>    TX:  5.60Mpps     | 5.70Mpps    | +1.7%
>    RX:  10.6Mpps     | 10.8Mpps    | +1.8%
> 
> Benchmark also shows no performance impact for in_order=off for queue
> size with 256 and 1024.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---

This is in linux-next now, but I get the following compilation error:

drivers/virtio/virtio_ring.c:2113:40: error: variable 'id' is uninitialized when used here [-Werror,-Wuninitialized]
 2113 |                 BAD_RING(vq, "id %u out of range\n", id);
      |                                                      ^~
drivers/virtio/virtio_ring.c:60:32: note: expanded from macro 'BAD_RING'
   60 |                         "%s:"fmt, (_vq)->vq.name, ##args);      \
      |                                                     ^~~~
./include/linux/dev_printk.h:154:65: note: expanded from macro 'dev_err'
  154 |         dev_printk_index_wrap(_dev_err, KERN_ERR, dev, dev_fmt(fmt), ##__VA_ARGS__)
      |                                                                        ^~~~~~~~~~~
./include/linux/dev_printk.h:110:23: note: expanded from macro 'dev_printk_index_wrap'
  110 |                 _p_func(dev, fmt, ##__VA_ARGS__);                       \
      |                                     ^~~~~~~~~~~
drivers/virtio/virtio_ring.c:2077:19: note: initialize the variable 'id' to silence this warning
 2077 |         u16 last_used, id, last_used_idx;
      |                          ^
      |                           = 0
1 error generated.

Thanks,
Thorsten