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Date:   Tue, 6 Jul 2021 11:22:41 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Yongji Xie <xieyongji@bytedance.com>
Cc:     Jason Wang <jasowang@redhat.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 11:04:18AM +0800, Yongji Xie wrote:
> On Mon, Jul 5, 2021 at 8:50 PM Stefan Hajnoczi <stefanha@redhat.com> wrot=
e:
> >
> > 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.
> > > > >
> > > > > 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.
> > >
> > >
> > > 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.  Virtqueue sho=
uld be
> > > much better. As Yong Ji quoted, the config space is used for
> > > "rarely-changing or intialization-time parameters".
> > >
> > >
> > > > 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.
> > > >
> > > > Jason and Stefan, what do you think of this way?
> >
> > Why does VDUSE_DEV_GET_CONFIG need to support an error return value?
> >
>=20
> We add a timeout and return error in case userspace never replies to
> the message.
>=20
> > The VIRTIO spec provides no way for the device to report errors from
> > config space accesses.
> >
> > The QEMU virtio-pci implementation returns -1 from invalid
> > virtio_config_read*() and silently discards virtio_config_write*()
> > accesses.
> >
> > VDUSE can take the same approach with
> > VDUSE_DEV_GET_CONFIG/VDUSE_DEV_SET_CONFIG.
> >
>=20
> I noticed that virtio_config_read*() only returns -1 when we access a
> invalid field. But in the VDUSE case, VDUSE_DEV_GET_CONFIG might fail
> when we access a valid field. Not sure if it's ok to silently ignore
> this kind of error.

That's a good point but it's a general VIRTIO issue. Any device
implementation (QEMU userspace, hardware vDPA, etc) can fail, so the
VIRTIO specification needs to provide a way for the driver to detect
this.

If userspace violates the contract then VDUSE needs to mark the device
broken. QEMU's device emulation does something similar with the
vdev->broken flag.

The VIRTIO Device Status field DEVICE_NEEDS_RESET bit can be set by
vDPA/VDUSE to indicate that the device is not operational and must be
reset.

The driver code may still process the -1 value read from the
configuration space. Hopefully this isn't a problem. There is currently
no VIRTIO interface besides DEVICE_NEEDS_RESET to indicate configuration
space access failures. On the other hand, drivers need to handle
malicious devices so they should be able to cope with the -1 value
anyway.

Stefan

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