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Date:   Fri, 27 Apr 2018 14:31:46 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     Stefan Hajnoczi <stefanha@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, qemu-devel@nongnu.org,
        linux-nvdimm@ml01.01.org, linux-mm@kvack.org, kwolf@redhat.com,
        haozhong zhang <haozhong.zhang@intel.com>, jack@suse.cz,
        xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
        riel@surriel.com, niteshnarayanlal@hotmail.com, david@redhat.com,
        ross zwisler <ross.zwisler@intel.com>, lcapitulino@redhat.com,
        hch@infradead.org, mst@redhat.com, imammedo@redhat.com,
        marcel@redhat.com, pbonzini@redhat.com,
        dan j williams <dan.j.williams@intel.com>, nilal@redhat.com
Subject: Re: [Qemu-devel] [RFC v2 1/2] virtio: add pmem driver
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On Thu, Apr 26, 2018 at 11:44:59AM -0400, Pankaj Gupta wrote:
> > > +	int err;
> > > +
> > > +	sg_init_one(&sg, buf, sizeof(buf));
> > > +
> > > +	err =3D 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);
> >=20
> > 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.
>=20
> I thought about it to some extent, and wanted to go ahead with simple ver=
sion first:
>=20
> - I think file 'inode -> locking' sill is there for request on single fil=
e.
> - 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.=20
>   We just want most recent flush to assure guest writes are synced proper=
ly.
>=20
> Important point here: We are doing entire block fsync for guest virtual d=
isk.

I don't understand your answer.  Is locking necessary or not?

=46rom the virtqueue_add_outbuf() documentation:

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

Stefan

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