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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

so buffer never overflows.

-- 
MST