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From: | Wei Wang |
Subject: | Re: [Qemu-devel] [virtio-dev] Re: [PATCH v11 6/6] virtio-balloon: VIRTIO_BALLOON_F_CMD_VQ |
Date: | Thu, 22 Jun 2017 16:40:39 +0800 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
On 06/21/2017 08:28 PM, Michael S. Tsirkin wrote:
On Wed, Jun 21, 2017 at 11:28:00AM +0800, Wei Wang wrote:On 06/21/2017 12:18 AM, Michael S. Tsirkin wrote:On Fri, Jun 09, 2017 at 06:41:41PM +0800, Wei Wang wrote:- if (!virtqueue_indirect_desc_table_add(vq, desc, num)) { + if (!virtqueue_indirect_desc_table_add(vq, desc, *num)) { virtqueue_kick(vq); - wait_event(vb->acked, virtqueue_get_buf(vq, &len)); - vb->balloon_page_chunk.chunk_num = 0; + if (busy_wait) + while (!virtqueue_get_buf(vq, &len) && + !virtqueue_is_broken(vq)) + cpu_relax(); + else + wait_event(vb->acked, virtqueue_get_buf(vq, &len));This is something I didn't previously notice. As you always keep a single buffer in flight, you do not really need indirect at all. Just add all descriptors in the ring directly, then kick. E.g. virtqueue_add_first virtqueue_add_next virtqueue_add_last ? You also want a flag to avoid allocations but there's no need to do it per descriptor, set it on vq.Without using the indirect table, I'm thinking about changing to use the standard sg (i.e. struct scatterlist), instead of vring_desc, so that we don't need to modify or add any new functions of virtqueue_add(). In this case, we will kmalloc an array of sgs in probe(), and we can add the sgs one by one to the vq, which won't trigger the allocation of an indirect table inside virtqueue_add(), and then kick when all are added. Best, WeiAnd allocate headers too? This can work. API extensions aren't necessarily a bad idea though. The API I suggest above is preferable for the simple reason that it can work without INDIRECT flag support in hypervisor.
OK, probably we don't need to add a desc to the vq - we can just use the vq's desc, like this: int virtqueue_add_first(struct virtqueue *_vq, uint64_t addr, uint32_t len, bool in, unsigned int *idx) { ... uint16_t desc_flags = in ? VRING_DESC_F_NEXT | VRING_DESC_F_WRITE : VRING_DESC_F_NEXT; vq->vring.desc[vq->free_head].addr = addr; vq->vring.desc[vq->free_head].len = len;vq->vring.desc[vq->free_head].flags = cpu_to_virtio16(_vq->vdev, flags);
/* return to the caller the desc id */ *idx = vq->free_head; ... } int virtqueue_add_next(struct virtqueue *_vq, uint64_t addr, uint32_t len, bool in, bool end, unsigned int *idx) { ... vq->vring.desc[*idx].next = vq->free_head; vq->vring.desc[vq->free_head].addr = addr; ... if (end) remove the VRING_DESC_F_NEXT flag } What do you think? We can also combine the two functions into one. Best, Wei
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