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Date:   Thu, 7 Jul 2022 01:59:15 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Guo Zhi <qtxuning1999@sjtu.edu.cn>
Cc:     jasowang@redhat.com, eperezma@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, sgarzare@redhat.com
Subject: Re: [PATCH v2 4/4] virtio_test: enable indirection feature
Message-ID: <20220707015717-mutt-send-email-mst@kernel.org>
References: <20220707024409.1869-1-qtxuning1999@sjtu.edu.cn>
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 <20220707005311-mutt-send-email-mst@kernel.org>
 <2ff418ad-0ab8-1474-4308-2d163c1e6652@sjtu.edu.cn>
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On Thu, Jul 07, 2022 at 01:56:37PM +0800, Guo Zhi wrote:
> On 2022/7/7 12:59, Michael S. Tsirkin wrote:
> > On Thu, Jul 07, 2022 at 10:44:09AM +0800, Guo Zhi wrote:
> > > Prior implementation don't use indirection feature because there is only
> > > one descriptor for every io event, actually prior implementation don't
> > > support indirection because vhost can't translate and find the indirect
> > > descriptors. This commit enable virtio_test malloc indirect descriptors
> > > in a indirect buffer and map this buffer to vhost, thus resolve this
> > > problem.
> > > 
> > > Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
> > > ---
> > >   tools/virtio/virtio_test.c | 50 ++++++++++++++++++++++++++++++++------
> > >   1 file changed, 42 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
> > > index 363695b33..dca408a5c 100644
> > > --- a/tools/virtio/virtio_test.c
> > > +++ b/tools/virtio/virtio_test.c
> > > @@ -25,7 +25,7 @@
> > >   #define RINGSIZE   256
> > >   #define TEST_BUF_NUM 0x100000
> > >   #define BUF_SIZE   1024
> > > -/* Unused */
> > > +#define INDIRECTS_SIZE   (RINGSIZE * sizeof(struct vring_desc) * 8)
> > >   void *__kmalloc_fake, *__kfree_ignore_start, *__kfree_ignore_end;
> > >   struct vq_info {
> > > @@ -47,6 +47,8 @@ struct vdev_info {
> > >   	int nvqs;
> > >   	void *buf;
> > >   	size_t buf_size;
> > > +	void *indirects;
> > > +	size_t indirects_size;
> > What are these exactly?
> The buffer is used to put indirect descriptors, and the buffer will be added
> in the vhost iotlb through VHOST_SET_MEM_TABLE, so that vhost can translate
> the descriptors in user mode correctly.

Pls come up with a better name. indirect_buf and indirect_buf_size?
And add a comment.

> > 
> > >   	struct vhost_memory *mem;
> > >   };
> > > @@ -131,6 +133,8 @@ static void vq_info_add(struct vdev_info *dev, int num)
> > >   static void vdev_info_init(struct vdev_info* dev, unsigned long long features)
> > >   {
> > >   	int r;
> > > +	int nregions = 2;
> > > +
> > >   	memset(dev, 0, sizeof *dev);
> > >   	dev->vdev.features = features;
> > >   	INIT_LIST_HEAD(&dev->vdev.vqs);
> > > @@ -138,19 +142,25 @@ static void vdev_info_init(struct vdev_info* dev, unsigned long long features)
> > >   	dev->buf_size = BUF_SIZE;
> > >   	dev->buf = malloc(dev->buf_size);
> > >   	assert(dev->buf);
> > > -        dev->control = open("/dev/vhost-test", O_RDWR);
> > > +	dev->indirects_size = INDIRECTS_SIZE;
> > > +	dev->indirects = malloc(dev->indirects_size);
> > > +	assert(dev->indirects);
> > > +	dev->control = open("/dev/vhost-test", O_RDWR);
> > >   	assert(dev->control >= 0);
> > >   	r = ioctl(dev->control, VHOST_SET_OWNER, NULL);
> > >   	assert(r >= 0);
> > >   	dev->mem = malloc(offsetof(struct vhost_memory, regions) +
> > > -			  sizeof dev->mem->regions[0]);
> > > +			(sizeof(dev->mem->regions[0])) * nregions);
> > >   	assert(dev->mem);
> > >   	memset(dev->mem, 0, offsetof(struct vhost_memory, regions) +
> > > -                          sizeof dev->mem->regions[0]);
> > > -	dev->mem->nregions = 1;
> > > +			(sizeof(dev->mem->regions[0])) * nregions);
> > > +	dev->mem->nregions = nregions;
> > >   	dev->mem->regions[0].guest_phys_addr = (long)dev->buf;
> > >   	dev->mem->regions[0].userspace_addr = (long)dev->buf;
> > >   	dev->mem->regions[0].memory_size = dev->buf_size;
> > > +	dev->mem->regions[1].guest_phys_addr = (long)dev->indirects;
> > > +	dev->mem->regions[1].userspace_addr = (long)dev->indirects;
> > > +	dev->mem->regions[1].memory_size = dev->indirects_size;
> > >   	r = ioctl(dev->control, VHOST_SET_MEM_TABLE, dev->mem);
> > >   	assert(r >= 0);
> > >   }
> > > @@ -170,6 +180,19 @@ static void wait_for_interrupt(struct vdev_info *dev)
> > >   		}
> > >   }
> > > +static int test_virtqueue_add_outbuf(struct virtqueue *vq,
> > > +				     struct scatterlist *sg, unsigned int num,
> > > +				     void *data, void *indirects)
> > > +{
> > > +	int r;
> > > +
> > > +	__kmalloc_fake = indirects;
> > > +	r = virtqueue_add_outbuf(vq, sg, num, data,
> > > +				 GFP_ATOMIC);
> > > +	__kmalloc_fake = NULL;
> > > +	return r;
> > > +}
> > > +
> > Quite a hack. Please add comments with documentation. Also - no way to
> > avoid hacks?
> 
> The __kmalloc_fake is refered from vringh_test.
> 
> If not using __kmalloc_fake here, the vhost doesn't know how to translate
> the indirects descriptors(user address).
> 
> We may could register a single region as large as the whole virtual space in
> the vhost iotlb using 1:1 mapping.
> 
> But since they are tests, IMHO, better here to use VHOST_SET_MEM_TABLE with
> more regions.

But why do we need to do this on each buffer add?

> > 
> > >   static void run_test(struct vdev_info *dev, struct vq_info *vq,
> > >   		     bool delayed, int batch, int reset_n, int bufs)
> > >   {
> > > @@ -181,6 +204,7 @@ static void run_test(struct vdev_info *dev, struct vq_info *vq,
> > >   	unsigned len;
> > >   	long long spurious = 0;
> > >   	const bool random_batch = batch == RANDOM_BATCH;
> > > +	void *indirects;
> > >   	r = ioctl(dev->control, VHOST_TEST_RUN, &test);
> > >   	assert(r >= 0);
> > > @@ -188,10 +212,15 @@ static void run_test(struct vdev_info *dev, struct vq_info *vq,
> > >   		next_reset = INT_MAX;
> > >   	}
> > > +	/* Don't kfree indirects. */
> > > +	__kfree_ignore_start = dev->indirects;
> > > +	__kfree_ignore_end = dev->indirects + dev->indirects_size;
> > > +
> > >   	for (;;) {
> > >   		virtqueue_disable_cb(vq->vq);
> > >   		completed_before = completed;
> > >   		started_before = started;
> > > +		indirects = dev->indirects;
> > >   		do {
> > >   			const bool reset = completed > next_reset;
> > >   			if (random_batch)
> > > @@ -203,9 +232,13 @@ static void run_test(struct vdev_info *dev, struct vq_info *vq,
> > >   				sg_init_table(sg, sg_size);
> > >   				for (int i = 0; i < sg_size; ++i)
> > >   					sg_set_buf(&sg[i], dev->buf + i, 0x1);
> > > -				r = virtqueue_add_outbuf(vq->vq, sg, sg_size,
> > > -							 dev->buf + started,
> > > -							 GFP_ATOMIC);
> > > +
> > > +				// use indirects buffer repeatedly
> > 
> > C style comments please.
> It will be modified.
> > 
> > > +				if (indirects + sg_size * sizeof(struct vring_desc) >
> > > +						dev->indirects + dev->indirects_size)
> > > +					indirects = dev->indirects;
> > > +				r = test_virtqueue_add_outbuf(vq->vq, sg, sg_size,
> > > +							      dev->buf + started, indirects);
> > >   				if (unlikely(r != 0)) {
> > >   					if (r == -ENOSPC &&
> > >   					    started > started_before)
> > > @@ -216,6 +249,7 @@ static void run_test(struct vdev_info *dev, struct vq_info *vq,
> > >   				}
> > >   				++started;
> > > +				indirects += sg_size * sizeof(struct vring_desc);
> > >   			}
> > >   			if (unlikely(!virtqueue_kick(vq->vq))) {
> > >   				r = -1;
> > > -- 
> > > 2.17.1
>