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

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Date:   Tue, 6 Nov 2018 14:47:30 -0500
From:   Vitaly Mayatskih <v.mayatskih@...il.com>
To:     stefanha@...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 Tue, Nov 6, 2018 at 11:03 AM Stefan Hajnoczi <stefanha@...il.com> wrote:

> 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.

Yes, vhost-user-blk was used as a template.

> > +enum {
> > +     VHOST_BLK_VQ_MAX = 16,
> > +     VHOST_BLK_VQ_MAX_REQS = 128,
> > +};
>
> These limits seem arbitrary and probably too low.

It fits cache and TLB, since the data structures are statically
allocated. I saw a worse performance with bigger max-reqs. I'll make
it configurable.

> > +     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.

Yeah, I'm making first baby steps in virtio ;) Thanks!

> > +     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.

vq->private_data is populated, vhost_set_vring_num returns -EBUSY if
ioctl is passed as is. It can be a bug in vhost, too, but I don't have
enough knowledge.

diff --git a/drivers/vhost/blk.c b/drivers/vhost/blk.c
index aefb9a61fa0f..da3eb041a975 100644
--- a/drivers/vhost/blk.c
+++ b/drivers/vhost/blk.c
@@ -438,7 +438,7 @@ static long vhost_blk_ioctl(struct file *f,
unsigned int ioctl,

        switch (ioctl) {
        case VHOST_SET_MEM_TABLE:
-               vhost_blk_stop(blk);
+//             vhost_blk_stop(blk);
                ret = vhost_blk_pass_ioctl(blk, ioctl, argp);
                break;
        case VHOST_SET_VRING_NUM:

./x86_64-softmmu/qemu-system-x86_64 --enable-kvm -cpu host -smp 4 -m
2G -vnc 192.168.122.104:5 --drive
if=none,id=drive0,format=raw,file=/dev/vde -device
vhost-blk-pci,id=blk0,drive=drive0,num-queues=4
qemu-system-x86_64: vhost_set_vring_num failed: Device or resource busy (16)
qemu-system-x86_64: Error starting vhost: 16

> > +     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)?

In vhost itself. I capture the parameter only if vhost ioctl completes
without errors. Perhaps, worth a comment.

Thanks for review, this is exactly what I hoping for!

--
wbr, Vitaly