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SubjectRe: [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct
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.


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