Re: [PATCH v2 4/4] virtio_test: enable indirection feature

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

On 2022/7/7 13:59, Michael S. Tsirkin wrote:
> 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.
Will be modified in next version batch.
>
>>>>    	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?
Because kmalloc in virtio_test will return address __kmalloc_fake. Since 
there are many descriptors batched here, It will be erase existed unused 
descriptors' indirect descriptors if using the same area for every 
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