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Message-ID: <b6f24555-aaf8-b470-ab16-099055364cfc@redhat.com>
Date: Mon, 14 Oct 2019 09:47:18 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC v1 2/2] vhost: batching fetches
On 2019/10/13 上午4:36, Michael S. Tsirkin wrote:
> On Sat, Oct 12, 2019 at 03:30:52PM +0800, Jason Wang wrote:
>> On 2019/10/11 下午9:46, Michael S. Tsirkin wrote:
>>> With this patch applied, new and old code perform identically.
>>>
>>> Lots of extra optimizations are now possible, e.g.
>>> we can fetch multiple heads with copy_from/to_user now.
>>> We can get rid of maintaining the log array. Etc etc.
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>> ---
>>> drivers/vhost/test.c | 2 +-
>>> drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++-------
>>> drivers/vhost/vhost.h | 4 +++-
>>> 3 files changed, 46 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
>>> index 39a018a7af2d..e3a8e9db22cd 100644
>>> --- a/drivers/vhost/test.c
>>> +++ b/drivers/vhost/test.c
>>> @@ -128,7 +128,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>>> dev = &n->dev;
>>> vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>>> n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
>>> - vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
>>> + vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>>> VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT);
>>> f->private_data = n;
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index 36661d6cb51f..aa383e847865 100644
>>> --- a/drivers/vhost/vhost.c
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -302,6 +302,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>> {
>>> vq->num = 1;
>>> vq->ndescs = 0;
>>> + vq->first_desc = 0;
>>> vq->desc = NULL;
>>> vq->avail = NULL;
>>> vq->used = NULL;
>>> @@ -390,6 +391,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>>> for (i = 0; i < dev->nvqs; ++i) {
>>> vq = dev->vqs[i];
>>> vq->max_descs = dev->iov_limit;
>>> + vq->batch_descs = dev->iov_limit - UIO_MAXIOV;
>>> vq->descs = kmalloc_array(vq->max_descs,
>>> sizeof(*vq->descs),
>>> GFP_KERNEL);
>>> @@ -2366,6 +2368,8 @@ static void pop_split_desc(struct vhost_virtqueue *vq)
>>> --vq->ndescs;
>>> }
>>> +#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
>>> + VRING_DESC_F_NEXT)
>>> static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
>>> {
>>> struct vhost_desc *h;
>>> @@ -2375,7 +2379,7 @@ static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc,
>>> h = &vq->descs[vq->ndescs++];
>>> h->addr = vhost64_to_cpu(vq, desc->addr);
>>> h->len = vhost32_to_cpu(vq, desc->len);
>>> - h->flags = vhost16_to_cpu(vq, desc->flags);
>>> + h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
>>> h->id = id;
>>> return 0;
>>> @@ -2450,7 +2454,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
>>> return 0;
>>> }
>>> -static int fetch_descs(struct vhost_virtqueue *vq)
>>> +static int fetch_buf(struct vhost_virtqueue *vq)
>>> {
>>> struct vring_desc desc;
>>> unsigned int i, head, found = 0;
>>> @@ -2462,7 +2466,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>>> /* Check it isn't doing very strange things with descriptor numbers. */
>>> last_avail_idx = vq->last_avail_idx;
>>> - if (vq->avail_idx == vq->last_avail_idx) {
>>> + if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
>>> + /* If we already have work to do, don't bother re-checking. */
>>> + if (likely(vq->ndescs))
>>> + return vq->num;
>>> +
>>> if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
>>> vq_err(vq, "Failed to access avail idx at %p\n",
>>> &vq->avail->idx);
>>> @@ -2541,6 +2549,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>>> return 0;
>>> }
>>> +static int fetch_descs(struct vhost_virtqueue *vq)
>>> +{
>>> + int ret = 0;
>>> +
>>> + if (unlikely(vq->first_desc >= vq->ndescs)) {
>>> + vq->first_desc = 0;
>>> + vq->ndescs = 0;
>>> + }
>>> +
>>> + if (vq->ndescs)
>>> + return 0;
>>> +
>>> + while (!ret && vq->ndescs <= vq->batch_descs)
>>> + ret = fetch_buf(vq);
>>
>> It looks to me descriptor chaining might be broken here.
> It should work because fetch_buf fetches a whole buf, following
> the chain. Seems to work in a small test ... what issues do you see?
Consider the case when fetch_buf return -EINVAL when push_split_desc()
fail. This means we only have partial descriptors.
Do we need decreasing last_avail_idx and vq->ndescs here? Or having a
head,tail instead of first_desc?
Thanks
>
>>> +
>>> + return vq->ndescs ? 0 : ret;
>>> +}
>>> +
>>> /* This looks in the virtqueue and for the first available buffer, and converts
>>> * it to an iovec for convenient access. Since descriptors consist of some
>>> * number of output then some number of input descriptors, it's actually two
>>> @@ -2562,6 +2588,8 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>> if (ret)
>>> return ret;
>>> + /* Note: indirect descriptors are not batched */
>>> + /* TODO: batch up to a limit */
>>> last = peek_split_desc(vq);
>>> id = last->id;
>>> @@ -2584,12 +2612,12 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>> if (unlikely(log))
>>> *log_num = 0;
>>> - for (i = 0; i < vq->ndescs; ++i) {
>>> + for (i = vq->first_desc; i < vq->ndescs; ++i) {
>>> unsigned iov_count = *in_num + *out_num;
>>> struct vhost_desc *desc = &vq->descs[i];
>>> int access;
>>> - if (desc->flags & ~(VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE)) {
>>> + if (desc->flags & ~VHOST_DESC_FLAGS) {
>>> vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
>>> desc->flags, desc->id);
>>> ret = -EINVAL;
>>> @@ -2628,15 +2656,21 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>> }
>>> *out_num += ret;
>>> }
>>> +
>>> + ret = desc->id;
>>> +
>>> + if (!(desc->flags & VRING_DESC_F_NEXT))
>>> + break;
>>> }
>>
>> What happens if we reach vq->ndescs but VRING_DESC_F_NEXT is still set?
>>
>> Thanks
> This can't happen: descriptors are pushed by push_split_desc each time
> we go through a loop in fetch_buf. The only way to exit the loop
> with return code 0 is if next_desc return -1 that is when VRING_DESC_F_NEXT
> is clear.
>
> But it's a good idea to add a BUG_ON here, I'll do it in the next version.
>
>
>>> - ret = id;
>>> - vq->ndescs = 0;
>>> + vq->first_desc = i + 1;
>>> return ret;
>>> err:
>>> - vhost_discard_vq_desc(vq, 1);
>>> + for (i = vq->first_desc; i < vq->ndescs; ++i)
>>> + if (!(desc->flags & VRING_DESC_F_NEXT))
>>> + vhost_discard_vq_desc(vq, 1);
>>> vq->ndescs = 0;
>>> return ret;
>>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>>> index 1724f61b6c2d..8b88e0c903da 100644
>>> --- a/drivers/vhost/vhost.h
>>> +++ b/drivers/vhost/vhost.h
>>> @@ -100,7 +100,9 @@ struct vhost_virtqueue {
>>> struct vhost_desc *descs;
>>> int ndescs;
>>> + int first_desc;
>>> int max_descs;
>>> + int batch_descs;
>>> const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
>>> struct file *kick;
>>> @@ -245,7 +247,7 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
>>> int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
>>> #define vq_err(vq, fmt, ...) do { \
>>> - pr_debug(pr_fmt(fmt), ##__VA_ARGS__); \
>>> + pr_err(pr_fmt(fmt), ##__VA_ARGS__); \
>>> if ((vq)->error_ctx) \
>>> eventfd_signal((vq)->error_ctx, 1);\
>>> } while (0)
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