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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sat, 12 Oct 2019 16:27:43 -0400

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 <[email-masked]>
> > ---
> >   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.