From: "Michael S. Tsirkin" <mst@redhat.com>
To: Si-Wei Liu <si-wei.liu@oracle.com>
Cc: Jason Wang <jasowang@redhat.com>,
elic@nvidia.com, linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
netdev@vger.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Sun, 28 Feb 2021 16:28:53 -0500 [thread overview]
Message-ID: <20210228162802-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <52836a63-4e00-ff58-50fb-9f450ce968d7@oracle.com>
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
next prev parent reply other threads:[~2021-02-28 21:30 UTC|newest]
Thread overview: 88+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 11:54 [PATCH] vdpa/mlx5: set_features should allow reset to zero Si-Wei Liu
2021-02-21 14:44 ` Eli Cohen
2021-02-21 21:52 ` Michael S. Tsirkin
2021-02-22 6:05 ` Eli Cohen
2021-02-23 9:26 ` Michael S. Tsirkin
2021-02-23 9:48 ` Jason Wang
2021-02-23 9:55 ` Michael S. Tsirkin
2021-02-22 4:14 ` Jason Wang
2021-02-22 7:34 ` Michael S. Tsirkin
2021-02-23 1:12 ` Si-Wei Liu
2021-02-23 2:03 ` Jason Wang
2021-02-23 13:26 ` Michael S. Tsirkin
2021-02-23 19:35 ` Si-Wei Liu
2021-02-24 3:20 ` Jason Wang
2021-02-24 5:17 ` Michael S. Tsirkin
2021-02-24 6:02 ` Jason Wang
2021-02-24 6:45 ` Eli Cohen
2021-02-24 6:47 ` Michael S. Tsirkin
2021-02-24 6:55 ` Jason Wang
2021-02-24 7:12 ` Michael S. Tsirkin
2021-02-24 12:40 ` Eli Cohen
2021-02-24 7:17 ` Eli Cohen
2021-02-24 5:04 ` Michael S. Tsirkin
2021-02-24 6:04 ` Jason Wang
2021-02-24 6:46 ` Michael S. Tsirkin
2021-02-24 6:53 ` Jason Wang
2021-02-24 7:17 ` Michael S. Tsirkin
[not found] ` <babc654d-8dcd-d8a2-c3b6-d20cc4fc554c@redhat.com>
2021-02-24 8:43 ` Michael S. Tsirkin
2021-02-24 9:30 ` Jason Wang
2021-02-28 21:30 ` Michael S. Tsirkin
2021-03-01 3:53 ` Jason Wang
2021-02-24 18:24 ` Si-Wei Liu
2021-02-26 0:56 ` Si-Wei Liu
2021-02-28 21:27 ` Michael S. Tsirkin
2021-03-01 18:08 ` Si-Wei Liu
2021-02-28 21:28 ` Michael S. Tsirkin [this message]
2021-02-28 21:34 ` Michael S. Tsirkin
2021-03-01 3:56 ` Jason Wang
2021-03-02 9:47 ` Michael S. Tsirkin
2021-03-02 10:53 ` Jason Wang
2021-12-11 1:44 ` vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero) Si-Wei Liu
2021-12-12 9:26 ` Michael S. Tsirkin
2021-12-13 3:02 ` Jason Wang
2021-12-13 8:06 ` Michael S. Tsirkin
2021-12-13 8:57 ` Jason Wang
2021-12-13 10:42 ` Michael S. Tsirkin
2021-12-14 1:13 ` Si-Wei Liu
2021-12-14 1:59 ` Si-Wei Liu
2021-12-14 3:01 ` Jason Wang
2021-12-14 5:06 ` Michael S. Tsirkin
2021-12-15 1:05 ` Si-Wei Liu
2021-12-15 2:06 ` Jason Wang
2021-12-15 20:52 ` Si-Wei Liu
2021-12-15 21:33 ` Michael S. Tsirkin
2021-12-16 2:01 ` Si-Wei Liu
2021-12-16 2:53 ` Jason Wang
2021-12-16 22:32 ` Si-Wei Liu
2021-12-17 1:57 ` Jason Wang
2021-12-17 2:00 ` Michael S. Tsirkin
2021-12-17 2:15 ` Jason Wang
2021-12-16 6:35 ` Michael S. Tsirkin
2021-12-16 3:43 ` Jason Wang
2021-12-17 1:08 ` Si-Wei Liu
2021-12-17 2:01 ` Jason Wang
2021-02-22 17:09 ` [PATCH] vdpa/mlx5: set_features should allow reset to zero Si-Wei Liu
2021-02-23 2:03 ` Jason Wang
2021-02-23 9:25 ` Michael S. Tsirkin
2021-02-23 9:46 ` Jason Wang
2021-02-23 10:01 ` Michael S. Tsirkin
2021-02-23 10:17 ` Jason Wang
2021-02-24 9:40 ` Jason Wang
2021-02-23 10:04 ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31 ` Jason Wang
2021-02-23 10:58 ` Cornelia Huck
2021-02-24 9:29 ` Jason Wang
2021-02-24 11:12 ` Cornelia Huck
2021-02-25 4:36 ` Jason Wang
2021-02-25 13:26 ` Cornelia Huck
2021-02-25 18:53 ` Michael S. Tsirkin
2021-02-26 8:19 ` Jason Wang
2021-02-28 21:25 ` Michael S. Tsirkin
2021-03-01 3:51 ` Jason Wang
2021-03-02 12:08 ` Cornelia Huck
[not found] ` <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com>
2021-03-03 8:29 ` Cornelia Huck
2021-03-04 8:24 ` Jason Wang
2021-03-04 13:50 ` Cornelia Huck
2021-03-05 3:01 ` Jason Wang
2021-02-23 12:26 ` Michael S. Tsirkin
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