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From: Jason Wang <jasowang@redhat.com>
Date: Mon, 18 Dec 2023 10:50:50 +0800
Message-ID: <CACGkMEtNZyHDzo2e_N9SM9dw4CK8vpZHWBrk7TFFixD1T7cdjw@mail.gmail.com>
Subject: Re: [PATCH v5 2/4] vduse: Temporarily disable control queue features
To: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: mst@redhat.com, xuanzhuo@linux.alibaba.com, paul@paul-moore.com, 
	jmorris@namei.org, serge@hallyn.com, stephen.smalley.work@gmail.com, 
	eparis@parisplace.org, xieyongji@bytedance.com, 
	virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, 
	linux-security-module@vger.kernel.org, selinux@vger.kernel.org, 
	david.marchand@redhat.com, lulu@redhat.com, casey@schaufler-ca.com
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On Wed, Dec 13, 2023 at 7:23=E2=80=AFPM Maxime Coquelin
<maxime.coquelin@redhat.com> wrote:
>
> Hi Jason,
>
> On 12/13/23 05:52, Jason Wang wrote:
> > On Tue, Dec 12, 2023 at 9:17=E2=80=AFPM Maxime Coquelin
> > <maxime.coquelin@redhat.com> wrote:
> >>
> >> Virtio-net driver control queue implementation is not safe
> >> when used with VDUSE. If the VDUSE application does not
> >> reply to control queue messages, it currently ends up
> >> hanging the kernel thread sending this command.
> >>
> >> Some work is on-going to make the control queue
> >> implementation robust with VDUSE. Until it is completed,
> >> let's disable control virtqueue and features that depend on
> >> it.
> >>
> >> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> >
> > I wonder if it's better to fail instead of a mask as a start.
>
> I think it is better to use a mask and not fail, so that we can in the
> future use a recent VDUSE application with an older kernel.

It may confuse the userspace unless userspace can do post check after
CREATE_DEV.

And for blk we fail when WCE is set in feature_is_valid():

static bool features_is_valid(u64 features)
{
        if (!(features & (1ULL << VIRTIO_F_ACCESS_PLATFORM)))
                return false;

        /* Now we only support read-only configuration space */
        if (features & (1ULL << VIRTIO_BLK_F_CONFIG_WCE))
                return false;

        return true;
}

Thanks

>
> Why would it be better to fail than negotiating?
>
> Thanks,
> Maxime
>