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Date:   Tue, 6 Jul 2021 11:14:51 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Yongji Xie <xieyongji@bytedance.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Stefano Garzarella <sgarzare@redhat.com>,
        Parav Pandit <parav@nvidia.com>,
        Christoph Hellwig <hch@infradead.org>,
        Christian Brauner <christian.brauner@canonical.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, bcrl@kvack.org,
        Jonathan Corbet <corbet@lwn.net>,
        Mika =?iso-8859-1?Q?Penttil=E4?= <mika.penttila@nextfour.com>,
        Dan Carpenter <dan.carpenter@oracle.com>, joro@8bytes.org,
        Greg KH <gregkh@linuxfoundation.org>, songmuchun@bytedance.com,
        virtualization <virtualization@lists.linux-foundation.org>,
        netdev@vger.kernel.org, kvm <kvm@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, iommu@lists.linux-foundation.org,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 10/10] Documentation: Add documentation for VDUSE
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On Tue, Jul 06, 2021 at 10:34:33AM +0800, Jason Wang wrote:
>=20
> =E5=9C=A8 2021/7/5 =E4=B8=8B=E5=8D=888:49, Stefan Hajnoczi =E5=86=99=E9=
=81=93:
> > On Mon, Jul 05, 2021 at 11:36:15AM +0800, Jason Wang wrote:
> > > =E5=9C=A8 2021/7/4 =E4=B8=8B=E5=8D=885:49, Yongji Xie =E5=86=99=E9=81=
=93:
> > > > > > OK, I get you now. Since the VIRTIO specification says "Device
> > > > > > configuration space is generally used for rarely-changing or
> > > > > > initialization-time parameters". I assume the VDUSE_DEV_SET_CON=
FIG
> > > > > > ioctl should not be called frequently.
> > > > > The spec uses MUST and other terms to define the precise requirem=
ents.
> > > > > Here the language (especially the word "generally") is weaker and=
 means
> > > > > there may be exceptions.
> > > > >=20
> > > > > Another type of access that doesn't work with the VDUSE_DEV_SET_C=
ONFIG
> > > > > approach is reads that have side-effects. For example, imagine a =
field
> > > > > containing an error code if the device encounters a problem unrel=
ated to
> > > > > a specific virtqueue request. Reading from this field resets the =
error
> > > > > code to 0, saving the driver an extra configuration space write a=
ccess
> > > > > and possibly race conditions. It isn't possible to implement those
> > > > > semantics suing VDUSE_DEV_SET_CONFIG. It's another corner case, b=
ut it
> > > > > makes me think that the interface does not allow full VIRTIO sema=
ntics.
> > >=20
> > > Note that though you're correct, my understanding is that config spac=
e is
> > > not suitable for this kind of error propagating. And it would be very=
 hard
> > > to implement such kind of semantic in some transports.=C2=A0 Virtqueu=
e should be
> > > much better. As Yong Ji quoted, the config space is used for
> > > "rarely-changing or intialization-time parameters".
> > >=20
> > >=20
> > > > Agreed. I will use VDUSE_DEV_GET_CONFIG in the next version. And to
> > > > handle the message failure, I'm going to add a return value to
> > > > virtio_config_ops.get() and virtio_cread_* API so that the error can
> > > > be propagated to the virtio device driver. Then the virtio-blk devi=
ce
> > > > driver can be modified to handle that.
> > > >=20
> > > > Jason and Stefan, what do you think of this way?
> > Why does VDUSE_DEV_GET_CONFIG need to support an error return value?
> >=20
> > The VIRTIO spec provides no way for the device to report errors from
> > config space accesses.
> >=20
> > The QEMU virtio-pci implementation returns -1 from invalid
> > virtio_config_read*() and silently discards virtio_config_write*()
> > accesses.
> >=20
> > VDUSE can take the same approach with
> > VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.
> >=20
> > > I'd like to stick to the current assumption thich get_config won't fa=
il.
> > > That is to say,
> > >=20
> > > 1) maintain a config in the kernel, make sure the config space read c=
an
> > > always succeed
> > > 2) introduce an ioctl for the vduse usersapce to update the config sp=
ace.
> > > 3) we can synchronize with the vduse userspace during set_config
> > >=20
> > > Does this work?
> > I noticed that caching is also allowed by the vhost-user protocol
> > messages (QEMU's docs/interop/vhost-user.rst), but the device doesn't
> > know whether or not caching is in effect. The interface you outlined
> > above requires caching.
> >=20
> > Is there a reason why the host kernel vDPA code needs to cache the
> > configuration space?
>=20
>=20
> Because:
>=20
> 1) Kernel can not wait forever in get_config(), this is the major differe=
nce
> with vhost-user.

virtio_cread() can sleep:

  #define virtio_cread(vdev, structname, member, ptr)                     \
          do {                                                            \
                  typeof(((structname*)0)->member) virtio_cread_v;        \
                                                                          \
                  might_sleep();                                          \
                  ^^^^^^^^^^^^^^

Which code path cannot sleep?

> 2) Stick to the current assumption that virtio_cread() should always
> succeed.

That can be done by reading -1 (like QEMU does) when the read fails.

Stefan

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