Re: [PATCH v13 05/13] vdpa: Add reset callback in vdpa_config_ops

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Yongji Xie <xieyongji@bytedance.com>
Cc: kvm <kvm@vger.kernel.org>,
	virtualization <virtualization@lists.linux-foundation.org>,
	"Christian Brauner" <christian.brauner@canonical.com>,
	"Will Deacon" <will@kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	joro@8bytes.org, "Matthew Wilcox" <willy@infradead.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Dan Carpenter" <dan.carpenter@oracle.com>,
	"John Garry" <john.garry@huawei.com>,
	"Liu Xiaodong" <xiaodong.liu@intel.com>,
	linux-fsdevel@vger.kernel.org,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	songmuchun@bytedance.com, "Jens Axboe" <axboe@kernel.dk>,
	"He Zhe" <zhe.he@windriver.com>,
	"Greg KH" <gregkh@linuxfoundation.org>,
	"Randy Dunlap" <rdunlap@infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	iommu@lists.linux-foundation.org, bcrl@kvack.org,
	netdev@vger.kernel.org, "Joe Perches" <joe@perches.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Mika Penttilä" <mika.penttila@nextfour.com>
Subject: Re: [PATCH v13 05/13] vdpa: Add reset callback in vdpa_config_ops
Date: Mon, 6 Sep 2021 02:37:03 -0400	[thread overview]
Message-ID: <20210906023131-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CACycT3v4ZVnh7DGe_RtAOx4Vvau0km=HWyCM=KzKhD+ahYKafQ@mail.gmail.com>

On Mon, Sep 06, 2021 at 02:09:25PM +0800, Yongji Xie wrote:
> On Mon, Sep 6, 2021 at 1:56 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Aug 31, 2021 at 06:36:26PM +0800, Xie Yongji wrote:
> > > This adds a new callback to support device specific reset
> > > behavior. The vdpa bus driver will call the reset function
> > > instead of setting status to zero during resetting.
> > >
> > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> >
> >
> > This does gloss over a significant change though:
> >
> >
> > > ---
> > > @@ -348,12 +352,12 @@ static inline struct device *vdpa_get_dma_dev(struct vdpa_device *vdev)
> > >       return vdev->dma_dev;
> > >  }
> > >
> > > -static inline void vdpa_reset(struct vdpa_device *vdev)
> > > +static inline int vdpa_reset(struct vdpa_device *vdev)
> > >  {
> > >       const struct vdpa_config_ops *ops = vdev->config;
> > >
> > >       vdev->features_valid = false;
> > > -     ops->set_status(vdev, 0);
> > > +     return ops->reset(vdev);
> > >  }
> > >
> > >  static inline int vdpa_set_features(struct vdpa_device *vdev, u64 features)
> >
> >
> > Unfortunately this breaks virtio_vdpa:
> >
> >
> > static void virtio_vdpa_reset(struct virtio_device *vdev)
> > {
> >         struct vdpa_device *vdpa = vd_get_vdpa(vdev);
> >
> >         vdpa_reset(vdpa);
> > }
> >
> >
> > and there's no easy way to fix this, kernel can't recover
> > from a reset failure e.g. during driver unbind.
> >
> 
> Yes, but it should be safe with the protection of software IOTLB even
> if the reset() fails during driver unbind.
> 
> Thanks,
> Yongji

Hmm. I don't see it.
What exactly will happen? What prevents device from poking at
memory after reset? Note that dma unmap in e.g. del_vqs happens
too late.  And what about e.g. interrupts?
E.g. we have this:

        /* Virtqueues are stopped, nothing can use vblk->vdev anymore. */
        vblk->vdev = NULL;

and this is no longer true at this point.

-- 
MST

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