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SubjectRe: [Qemu-devel] [RFC v2 1/2] virtio: add pmem driver

> > > > + int err;
> > > > +
> > > > + sg_init_one(&sg, buf, sizeof(buf));
> > > > +
> > > > + err = virtqueue_add_outbuf(vpmem->req_vq, &sg, 1, buf, GFP_KERNEL);
> > > > +
> > > > + if (err) {
> > > > + dev_err(&vdev->dev, "failed to send command to virtio pmem
> > > > device\n");
> > > > + return;
> > > > + }
> > > > +
> > > > + virtqueue_kick(vpmem->req_vq);
> > >
> > > Is any locking necessary? Two CPUs must not invoke virtio_pmem_flush()
> > > at the same time. Not sure if anything guarantees this, maybe you're
> > > relying on libnvdimm but I haven't checked.
> >
> > I thought about it to some extent, and wanted to go ahead with simple
> > version first:
> >
> > - I think file 'inode -> locking' sill is there for request on single file.
> > - For multiple files, our aim is to just flush the backend block image.
> > - Even there is collision for virt queue read/write entry it should just
> > trigger a Qemu fsync.
> > We just want most recent flush to assure guest writes are synced
> > properly.
> >
> > Important point here: We are doing entire block fsync for guest virtual
> > disk.
>
> I don't understand your answer. Is locking necessary or not?

It will be required with other changes.

>
> From the virtqueue_add_outbuf() documentation:
>
> * Caller must ensure we don't call this with other virtqueue operations
> * at the same time (except where noted).

Yes, I also saw it. But thought if can avoid it with current functionality. :)


Thanks,
Pankaj

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