linux-kernel - Re: [PATCH 1/1] Add vhost

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Date:   Tue, 6 Nov 2018 16:03:16 +0000
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Vitaly Mayatskikh <v.mayatskih@...il.com>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Add vhost_blk driver

On Fri, Nov 02, 2018 at 06:21:23PM +0000, Vitaly Mayatskikh wrote:
> This driver accelerates host side of virtio-blk.

Did you look at vhost-user-blk?  It does things slightly differently:
more of the virtio-blk device model is handled by the vhost-user device
(e.g. config space).  That might be necessary to implement
virtio_blk_config.writeback properly.

> +#define VHOST_BLK_SET_BACKEND _IOW(VHOST_VIRTIO, 0x50, int)

Needs to be in the uapi header so userspace can use it.

> +
> +enum {
> +	VHOST_BLK_VQ_MAX = 16,
> +	VHOST_BLK_VQ_MAX_REQS = 128,
> +};

These limits seem arbitrary and probably too low.

> +
> +struct vhost_blk_req {
> +	struct llist_node list;
> +	int index;
> +	struct vhost_blk_queue *q;
> +	struct virtio_blk_outhdr hdr;
> +	struct iovec *out_iov;
> +	struct iovec *in_iov;
> +	u8 out_num;
> +	u8 in_num;
> +	long len;
> +	struct kiocb iocb;
> +	struct iov_iter i;
> +	int res;
> +	void __user *status;
> +};
> +
> +struct vhost_blk_queue {
> +	int index;
> +	struct vhost_blk *blk;
> +	struct vhost_virtqueue vq;
> +	struct vhost_work w;
> +	struct llist_head wl;

What is this?  Please use clear names and comments. :)

> +static int vhost_blk_req_handle(struct vhost_blk_req *req)
> +{
> +	struct vhost_blk *blk = req->q->blk;
> +	struct vhost_virtqueue *vq = &req->q->vq;
> +	int type = le32_to_cpu(req->hdr.type);
> +	int ret;
> +	u8 status;
> +
> +	if ((type == VIRTIO_BLK_T_IN) || (type == VIRTIO_BLK_T_OUT)) {
> +		bool write = (type == VIRTIO_BLK_T_OUT);
> +		int nr_seg = (write ? req->out_num : req->in_num) - 1;
> +		unsigned long sector = le64_to_cpu(req->hdr.sector);

Using little-endian instead of the virtio types means that only VIRTIO
1.0 modern devices are supported (older devices may not be
little-endian!).  In that case you'd need to put the VIRTIO_1 feature
bit into the features mask.

> +		req = &q->req[head];
> +		req->index = head;
> +		req->out_num = out;
> +		req->in_num = in;
> +		req->out_iov = &vq->iov[1];
> +		req->in_iov = &vq->iov[out];
> +		req->status = vq->iov[out + in - 1].iov_base;

The VIRTIO spec explicitly requires that devices support arbitrary
descriptor layouts, so you cannot assume a particular iov[] layout.

> +static long vhost_blk_ioctl(struct file *f, unsigned int ioctl,
> +			    unsigned long arg)
> +{
> +	struct vhost_blk *blk = f->private_data;
> +	void __user *argp = (void __user *)arg;
> +	int fd;
> +	u64 __user *featurep = argp;
> +	u64 features;
> +	long ret;
> +	struct vhost_vring_state s;
> +
> +	switch (ioctl) {
> +	case VHOST_SET_MEM_TABLE:
> +		vhost_blk_stop(blk);
> +		ret = vhost_blk_pass_ioctl(blk, ioctl, argp);
> +		break;

Why is this necessary?  Existing vhost devices pass the ioctl through
without an explicit case for it.

> +	case VHOST_SET_VRING_NUM:
> +		if (copy_from_user(&s, argp, sizeof(s)))
> +			return -EFAULT;
> +		ret = vhost_blk_pass_ioctl(blk, ioctl, argp);
> +		if (!ret)
> +			blk->num_queues = s.index + 1;

Where is this input checked against ARRAY_SIZE(blk->queue)?

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