Subject: Re: [PATCH vhost v12 10/10] virtio_net: merge dma operations when filling mergeable buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Jul 2023 07:05:50 -0400

On Wed, Jul 19, 2023 at 06:33:05PM +0800, kernel test robot wrote:
> Hi Xuan,
> 
> kernel test robot noticed the following build warnings:
> 
> [auto build test WARNING on v6.4]
> [cannot apply to mst-vhost/linux-next linus/master v6.5-rc2 v6.5-rc1 next-20230719]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Xuan-Zhuo/virtio_ring-check-use_dma_api-before-unmap-desc-for-indirect/20230719-121424
> base:   v6.4
> patch link:    https://lore.kernel.org/r/20230719040422.126357-11-xuanzhuo%40linux.alibaba.com
> patch subject: [PATCH vhost v12 10/10] virtio_net: merge dma operations when filling mergeable buffers
> config: i386-randconfig-i006-20230718 (https://download.01.org/0day-ci/archive/20230719/[email-masked]/config)
> compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
> reproduce: (https://download.01.org/0day-ci/archive/20230719/[email-masked]/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <[email-masked]>
> | Closes: https://lore.kernel.org/oe-kbuild-all/[email-masked]/
> 
> All warnings (new ones prefixed by >>):
> 
>    drivers/net/virtio_net.c: In function 'virtnet_rq_init_one_sg':
> >> drivers/net/virtio_net.c:624:41: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
>      624 |                 rq->sg[0].dma_address = (dma_addr_t)addr;
>          |                                         ^
>    drivers/net/virtio_net.c: In function 'virtnet_rq_alloc':
> >> drivers/net/virtio_net.c:682:28: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
>      682 |                 *sg_addr = (void *)(dma->addr + alloc_frag->offset - sizeof(*dma));
>          |                            ^

yea these casts are pretty creepy. I think it's possible dma_addr_t won't fit in a pointer
or a pointer won't fit in dma_addr_t.

> 
> vim +624 drivers/net/virtio_net.c
> 
>    619	
>    620	static void virtnet_rq_init_one_sg(struct receive_queue *rq, void *addr, u32 len)
>    621	{
>    622		if (rq->do_dma) {
>    623			sg_init_table(rq->sg, 1);
>  > 624			rq->sg[0].dma_address = (dma_addr_t)addr;
>    625			rq->sg[0].length = len;
>    626		} else {
>    627			sg_init_one(rq->sg, addr, len);
>    628		}
>    629	}
>    630	
>    631	static void *virtnet_rq_alloc(struct receive_queue *rq, u32 size,
>    632				      void **sg_addr, gfp_t gfp)
>    633	{
>    634		struct page_frag *alloc_frag = &rq->alloc_frag;
>    635		struct virtnet_rq_dma *dma;
>    636		struct device *dev;
>    637		void *buf, *head;
>    638		dma_addr_t addr;
>    639	
>    640		if (unlikely(!skb_page_frag_refill(size, alloc_frag, gfp)))
>    641			return NULL;
>    642	
>    643		head = (char *)page_address(alloc_frag->page);
>    644	
>    645		if (rq->do_dma) {
>    646			dma = head;
>    647	
>    648			/* new pages */
>    649			if (!alloc_frag->offset) {
>    650				if (rq->last_dma) {
>    651					/* Now, the new page is allocated, the last dma
>    652					 * will not be used. So the dma can be unmapped
>    653					 * if the ref is 0.
>    654					 */
>    655					virtnet_rq_unmap(rq, rq->last_dma, 0);
>    656					rq->last_dma = NULL;
>    657				}
>    658	
>    659				dev = virtqueue_dma_dev(rq->vq);
>    660	
>    661				dma->len = alloc_frag->size - sizeof(*dma);
>    662	
>    663				addr = dma_map_single_attrs(dev, dma + 1, dma->len, DMA_FROM_DEVICE, 0);
>    664				if (addr == DMA_MAPPING_ERROR)
>    665					return NULL;
>    666	
>    667				dma->addr = addr;
>    668				dma->need_sync = dma_need_sync(dev, addr);
>    669	
>    670				/* Add a reference to dma to prevent the entire dma from
>    671				 * being released during error handling. This reference
>    672				 * will be freed after the pages are no longer used.
>    673				 */
>    674				get_page(alloc_frag->page);
>    675				dma->ref = 1;
>    676				alloc_frag->offset = sizeof(*dma);
>    677	
>    678				rq->last_dma = dma;
>    679			}
>    680	
>    681			++dma->ref;
>  > 682			*sg_addr = (void *)(dma->addr + alloc_frag->offset - sizeof(*dma));
>    683		} else {
>    684			*sg_addr = head + alloc_frag->offset;
>    685		}
>    686	
>    687		buf = head + alloc_frag->offset;
>    688	
>    689		get_page(alloc_frag->page);
>    690		alloc_frag->offset += size;
>    691	
>    692		return buf;
>    693	}
>    694	
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki