Date:   Sun, 28 Feb 2021 16:28:53 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     Jason Wang <jasowang@...hat.com>, elic@...dia.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote:
> 
> 
> On 2/23/2021 9:04 PM, Michael S. Tsirkin wrote:
> > On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> > > 
> > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, Jason Wang wrote:
> > > > > > > > On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
> > > > > > > > > Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
> > > > > > > > > for legacy") made an exception for legacy guests to reset
> > > > > > > > > features to 0, when config space is accessed before features
> > > > > > > > > are set. We should relieve the verify_min_features() check
> > > > > > > > > and allow features reset to 0 for this case.
> > > > > > > > > 
> > > > > > > > > It's worth noting that not just legacy guests could access
> > > > > > > > > config space before features are set. For instance, when
> > > > > > > > > feature VIRTIO_NET_F_MTU is advertised some modern driver
> > > > > > > > > will try to access and validate the MTU present in the config
> > > > > > > > > space before virtio features are set.
> > > > > > > > This looks like a spec violation:
> > > > > > > > 
> > > > > > > > "
> > > > > > > > 
> > > > > > > > The following driver-read-only field, mtu only exists if
> > > > > > > > VIRTIO_NET_F_MTU is
> > > > > > > > set.
> > > > > > > > This field specifies the maximum MTU for the driver to use.
> > > > > > > > "
> > > > > > > > 
> > > > > > > > Do we really want to workaround this?
> > > > > > > > 
> > > > > > > > Thanks
> > > > > > > And also:
> > > > > > > 
> > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > 1. Reset the device.
> > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > device.
> > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > understood by the OS and driver to the
> > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > the device-specific configuration
> > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > feature bits after this step.
> > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > otherwise, the device does not
> > > > > > > support our subset of features and the device is unusable.
> > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > for the device, optional per-bus setup,
> > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > space, and population of virtqueues.
> > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > 
> > > > > > > 
> > > > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > thought the legacy guest still needs to be supported.
> > > > > > 
> > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > 
> > > > > > -Siwei
> > > > > I think it's a bug to read config space in validate, we should move it to
> > > > > virtnet_probe().
> > > > > 
> > > > > Thanks
> > > > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > the best way to handle this.
> > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> > > guest? Supposedly only config space write access needs to be guarded before
> > > setting FEATURES_OK.
> > > 
> > > -Siwie
> > Detecting it isn't enough though, we will need a new ioctl to notify
> > the kernel that it's a legacy guest. Ugh :(
> Well, although I think adding an ioctl is doable, may I know what the use
> case there will be for kernel to leverage such info directly? Is there a
> case QEMU can't do with dedicate ioctls later if there's indeed
> differentiation (legacy v.s. modern) needed?
> 
> One of the reason I asked is if this ioctl becomes a mandate for vhost-vdpa
> kernel. QEMU would reject initialize vhost-vdpa if doesn't see this ioctl
> coming?

Only on BE hosts or guests I think. With LE host and guest legacy and
modern behave the same so ioctl isn't needed.

> If it's optional, suppose the kernel may need it only when it becomes
> necessary?
> 
> Thanks,
> -Siwei

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