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

[Jason Wang]

On 2019/10/16 äå4:20, Michael S. Tsirkin wrote:
> On Mon, Oct 14, 2019 at 09:43:25AM +0800, Jason Wang wrote:
> > On 2019/10/13 äå4:27, Michael S. Tsirkin wrote:
> > > On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote:
> > > > On 2019/10/11 äå9:45, 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@xxxxxxxxxx>
> > > > > ---
> > > > >     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.