linux-kernel - Re: [Qemu-devel] [RFC v2 1/2] virtio: add pmem driver

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Date:   Sat, 28 Apr 2018 06:48:41 -0400 (EDT)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     jack@...e.cz, kvm@...r.kernel.org, david@...hat.com,
        linux-nvdimm@...1.01.org, ross zwisler <ross.zwisler@...el.com>,
        qemu-devel@...gnu.org, lcapitulino@...hat.com, linux-mm@...ck.org,
        niteshnarayanlal@...mail.com, mst@...hat.com, hch@...radead.org,
        Stefan Hajnoczi <stefanha@...il.com>, marcel@...hat.com,
        nilal@...hat.com, haozhong zhang <haozhong.zhang@...el.com>,
        riel@...riel.com, pbonzini@...hat.com,
        dan j williams <dan.j.williams@...el.com>, kwolf@...hat.com,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        linux-kernel@...r.kernel.org, imammedo@...hat.com
Subject: Re: [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