Re: [PATCH net-next v5 1/4] virtio_ring: enable premapped mode whatever use_dma_api

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Hi,

May I know if this is really an intended fix to post officially, or just a workaround/probe to make the offset in page_frag happy when net_high_order_alloc_disable is true? In case it's the former, even though this could fix the issue, I would assume clamping to a smaller page_frag than a regular page size for every buffer may have certain performance regression for the merge-able buffer case? Can you justify the performance impact with some benchmark runs with larger MTU and merge-able rx buffers to prove the regression is negligible? You would need to compare against where you don't have the inadvertent virtnet_rq_dma cost on any page i.e. getting all 4 patches of this series reverted. Both tests with net_high_order_alloc_disable set to on and off are needed.

Thanks,
-Siwei

On 8/17/2024 6:20 AM, Xuan Zhuo wrote:
Hi, guys, I have a fix patch for this.
Could anybody test it?

Thanks.

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index af474cc191d0..426d68c2d01d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2492,13 +2492,15 @@ static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
  {
         struct virtnet_info *vi = rq->vq->vdev->priv;
         const size_t hdr_len = vi->hdr_len;
-       unsigned int len;
+       unsigned int len, max_len;
+
+       max_len = PAGE_SIZE - ALIGN(sizeof(struct virtnet_rq_dma), L1_CACHE_BYTES);

         if (room)
-               return PAGE_SIZE - room;
+               return max_len - room;

         len = hdr_len + clamp_t(unsigned int, ewma_pkt_len_read(avg_pkt_len),
-                               rq->min_buf_len, PAGE_SIZE - hdr_len);
+                               rq->min_buf_len, max_len - hdr_len);

         return ALIGN(len, L1_CACHE_BYTES);
  }





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