Messages in this thread | | | Subject | Re: [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct | From | Jason Wang <> | Date | Wed, 16 Oct 2019 12:38:15 +0800 |
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On 2019/10/16 上午4:20, Michael S. Tsirkin wrote: > On Mon, Oct 14, 2019 at 09:43:25AM +0800, Jason Wang wrote: >> On 2019/10/13 上午4:27, Michael S. Tsirkin wrote: >>> On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote: >>>> On 2019/10/11 下午9:45, Michael S. Tsirkin wrote: >>>>> The idea is to support multiple ring formats by converting >>>>> to a format-independent array of descriptors. >>>>> >>>>> This costs extra cycles, but we gain in ability >>>>> to fetch a batch of descriptors in one go, which >>>>> is good for code cache locality. >>>>> >>>>> To simplify benchmarking, I kept the old code >>>>> around so one can switch back and forth by >>>>> writing into a module parameter. >>>>> This will go away in the final submission. >>>>> >>>>> This patch causes a minor performance degradation, >>>>> it's been kept as simple as possible for ease of review. >>>>> Next patch gets us back the performance by adding batching. >>>>> >>>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> >>>>> --- >>>>> drivers/vhost/test.c | 17 ++- >>>>> drivers/vhost/vhost.c | 299 +++++++++++++++++++++++++++++++++++++++++- >>>>> drivers/vhost/vhost.h | 16 +++ >>>>> 3 files changed, 327 insertions(+), 5 deletions(-) >>>>> >>>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c >>>>> index 056308008288..39a018a7af2d 100644 >>>>> --- a/drivers/vhost/test.c >>>>> +++ b/drivers/vhost/test.c >>>>> @@ -18,6 +18,9 @@ >>>>> #include "test.h" >>>>> #include "vhost.h" >>>>> +static int newcode = 0; >>>>> +module_param(newcode, int, 0644); >>>>> + >>>>> /* Max number of bytes transferred before requeueing the job. >>>>> * Using this limit prevents one virtqueue from starving others. */ >>>>> #define VHOST_TEST_WEIGHT 0x80000 >>>>> @@ -58,10 +61,16 @@ static void handle_vq(struct vhost_test *n) >>>>> vhost_disable_notify(&n->dev, vq); >>>>> for (;;) { >>>>> - head = vhost_get_vq_desc(vq, vq->iov, >>>>> - ARRAY_SIZE(vq->iov), >>>>> - &out, &in, >>>>> - NULL, NULL); >>>>> + if (newcode) >>>>> + head = vhost_get_vq_desc_batch(vq, vq->iov, >>>>> + ARRAY_SIZE(vq->iov), >>>>> + &out, &in, >>>>> + NULL, NULL); >>>>> + else >>>>> + head = vhost_get_vq_desc(vq, vq->iov, >>>>> + ARRAY_SIZE(vq->iov), >>>>> + &out, &in, >>>>> + NULL, NULL); >>>>> /* On error, stop handling until the next kick. */ >>>>> if (unlikely(head < 0)) >>>>> break; >>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >>>>> index 36ca2cf419bf..36661d6cb51f 100644 >>>>> --- a/drivers/vhost/vhost.c >>>>> +++ b/drivers/vhost/vhost.c >>>>> @@ -301,6 +301,7 @@ static void vhost_vq_reset(struct vhost_dev *dev, >>>>> struct vhost_virtqueue *vq) >>>>> { >>>>> vq->num = 1; >>>>> + vq->ndescs = 0; >>>>> vq->desc = NULL; >>>>> vq->avail = NULL; >>>>> vq->used = NULL; >>>>> @@ -369,6 +370,9 @@ static int vhost_worker(void *data) >>>>> static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq) >>>>> { >>>>> + kfree(vq->descs); >>>>> + vq->descs = NULL; >>>>> + vq->max_descs = 0; >>>>> kfree(vq->indirect); >>>>> vq->indirect = NULL; >>>>> kfree(vq->log); >>>>> @@ -385,6 +389,10 @@ 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->descs = kmalloc_array(vq->max_descs, >>>>> + sizeof(*vq->descs), >>>>> + GFP_KERNEL); >>>> Is iov_limit too much here? It can obviously increase the footprint. I guess >>>> the batching can only be done for descriptor without indirect or next set. >>>> Then we may batch 16 or 64. >>>> >>>> Thanks >>> Yes, next patch only batches up to 64. But we do need iov_limit because >>> guest can pass a long chain of scatter/gather. >>> We already have iovecs in a huge array so this does not look like >>> a big deal. If we ever teach the code to avoid the huge >>> iov arrays by handling huge s/g lists piece by piece, >>> we can make the desc array smaller at the same point. >>> >> Another possible issue, if we try to batch descriptor chain when we've >> already batched some descriptors, we may reach the limit then some of the >> descriptors might need re-read. >> >> Or we may need circular index (head, tail) in this case? >> >> Thanks > We never supported more than IOV_MAX descriptors. > And we don't batch more than iov_limit - IOV_MAX.
Ok, but what happens when we've already batched 63 descriptors then come a 3 descriptor chain?
And it looks to me we need forget the cached descriptor during set_vring_base()
Thanks
> > so buffer never overflows. >
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