Re: [virtio-comment] [PATCH v10 02/10] admin: introduce device group and related concepts

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 13, 2023 at 12:49:44AM +0200, Max Gurtovoy wrote:
> 
> 
> On 12/02/2023 22:19, Michael S. Tsirkin wrote:
> > On Sun, Feb 12, 2023 at 04:34:09PM +0200, Max Gurtovoy wrote:
> > > 
> > > 
> > > On 12/02/2023 15:15, Michael S. Tsirkin wrote:
> > > > On Sun, Feb 12, 2023 at 02:10:29PM +0200, Max Gurtovoy wrote:
> > > > > 
> > > > > 
> > > > > On 09/02/2023 21:58, Michael S. Tsirkin wrote:
> > > > > > On Thu, Feb 09, 2023 at 07:47:56PM +0200, Max Gurtovoy wrote:
> > > > > > > 
> > > > > > > On 09/02/2023 17:22, David Edmondson wrote:
> > > > > > > 
> > > > > > >        On Thursday, 2023-02-09 at 10:13:46 -05, Michael S. Tsirkin wrote:
> > > > > > > 
> > > > > > >            On Thu, Feb 09, 2023 at 03:00:58PM +0000, David Edmondson wrote:
> > > > > > > 
> > > > > > >                On Thursday, 2023-02-09 at 07:13:36 -05, Michael S. Tsirkin wrote:
> > > > > > > 
> > > > > > >                    Each device group has a type. For now, define one initial group:
> > > > > > > 
> > > > > > >                    SR-IOV type - PCI SR-IOV virtual functions (VFs) of a given
> > > > > > >                    PCI SR-IOV physical function (PF). This group may contain one or more
> > > > > > >                    virtio devices.
> > > > > > > 
> > > > > > >                The specification says zero or more generally and nothing to refine that
> > > > > > >                for SR-IOV groups.
> > > > > > > 
> > > > > > >            Well it says:
> > > > > > > 
> > > > > > >                    (SR-IOV) physical function (PF) device as the owner and includes
> > > > > > >                    all its SR-IOV virtual functions (VFs) as members (see
> > > > > > >                    \hyperref[intro:PCIe]{[PCIe]}).
> > > > > > > 
> > > > > > >            how can that be zero?
> > > > > > > 
> > > > > > >        If I remove all of the VFs of a PF, does the group implicitly disappear?
> > > > > > > 
> > > > > > > Enable/Disable SR-IOV are dynamic operations controlled by NumVFs register.
> > > > > > 
> > > > > > Are you sure? Not by VF Enable?
> > > > > 
> > > > > Right. It's a combination of the VF_Enable and numVfs.
> > > > > 
> > > > > ...
> > > > > "The NumVFs field defines the number of VFs that are enabled when VF Enable
> > > > > is Set in the associated PF."
> > > > > 
> > > > > > 
> > > > > > > We can mention that a group include all virtual functions indicated by TotalVFs
> > > > > > > RO register.
> > > > > > 
> > > > > > TotalVFs indicates the maximum number of VFs that could be associated with the PF
> > > > > > but not all of these exist. If we use this we get into issues of
> > > > > > discovering whether VFs can actually be used. I'd rather avoid that.
> > > > > > 
> > > > > > 
> > > > > > > This group exist as long as the owner PF supports SR-IOV and has admin
> > > > > > > capabilities.
> > > > > > 
> > > > > > SRIOV spec says:
> > > > > > 	NumVFs controls the number of VFs that are visible.
> > > > > > 
> > > > > > And one can not play with NumVFs dynamically:
> > > > > > 
> > > > > > 	NumVFs may only be written while VF Enable is Clear. If NumVFs is written when VF Enable is
> > > > > > 	Set, the results are undefined.
> > > > > > 
> > > > > > one has to kill all VFs then enable them all again.
> > > > > > As long as we are doing that, we can just unload the driver.
> > > > > what driver to unload ? Not sure I'm following..
> > > > 
> > > > Basically I am saying that if VFs are disabled this destroys all VFs
> > > > so it is not
> > > > clear that it makes sense to keep some of the configuration
> > > > for previous VFs around.
> > > > 
> > > > My idea is to say that reset is required to disable VFs.
> > > 
> > > which reset ?
> > > 
> > > VFs disabled/enabled using VF_enable PCI register.
> > > Lets follow that please.
> > > 
> > > Lets define what is happening during VF_enable = 1 and VF_enable = 0.
> > > 
> > > On VF_enable = 0 --> 1 transition the corresponding device_group, that is
> > > owned by the PF, grows and contains NumVFs members with initial features and
> > > resources.
> > > 
> > > On VF_enable = 1 --> 0 transition the corresponding device_group, that is
> > > owned by the PF, shrinks and contains 0 members and all the dynamic
> > > resources are back to the owner.
> > 
> > Maybe, it's a possible approach. As current patch set has no
> > resources I feel we can leave this part to a patch on top
> > just so that this one can move forward while we have
> > a more specific example to work from.
> 
> we can differ the resources part but lets add the part that the group
> becomes empty/non-empty according to the disable/enable of VF_enable
> register as mentioned above.

Hmm.  Another option is VIRTIO_ADMIN_STATUS_Q_INVALID_GROUP when VF_enable
is clear - since this disables the whole group not a specific member.
This allows distinguishing between id > NumVFs and VF_enable == 0
so it seems like a nicer option to me.


> > 
> > 
> > 
> > > > 
> > > > > > 
> > > > > > So I would say we should just ask that VF Enable is set
> > > > > > before poking at admin vq, and prohibit clearing VF Enable.
> > > > > > 
> > > > > 
> > > > > Sending admin commands shouldn't be restricted and should be allowed always.
> > > > > Device should react to incoming command.
> > > > > 
> > > > > We can say that an SR-IOV group is empty while the VF enable is unset and is
> > > > > not empty when it's set. The size of the non empty SR-IOV group is equal to
> > > > > the numVFs and the VF index is equal to the member identifier in this group.
> > > > > I had this definition in my patches.
> > > > > 
> > > > > Sending admin command to a non existing member will result in an ERROR
> > > > > VIRTIO_ADMIN_STATUS_Q_INVALID_MEM.
> > > > 
> > > > 
> > > > just an example:
> > > > - assign resource X to VF 16
> > > > - set NumVFs to 8
> > > > 
> > > > Does VF 16 keep resource X? I note that
> > > > resource X can not be taken back because VF 16
> > > > is not accessible.
> > > 
> > > all dynamic resources should return to the owner when VF_enable moves 1 -->
> > > 0
> > 
> > 
> > It's a possible approach but my question would be whether we
> > need a transport independent way to do this through VQ.
> > And if we do it's annoying to have multiple ways.
> 
> We are defining the existence of the SR-IOV group.
> 
> Also, the approach of sending commands to a non-existing member is valid for
> any group type and should return error VIRTIO_ADMIN_STATUS_Q_INVALID_MEM in
> that case.
> 
> why do we need more ways ?

Sorry I mean a transport independent way to destroy a group as a whole.


> > 
> > > > 
> > > > 
> > > > For sure, all this is solvable but I'd rather just forbid
> > > > changes to VF enable for now.
> > > 
> > > how we can forbid it ? setting this register is not related to virtio.
> > 
> > We can ask driver to write 0 to device reset before changing VF
> > enable to 0. The advantage is that this kind of reset by definition
> > frees up all resources.
> 
> why do we need to reset the device ? It is fully functional and should be
> able to run perfectly fine without any need to reset.
> 
> The PF device might have a very big role in the hypervisor, so restricting
> it in this way doesn't sounds mandatory.
> 
> Controlling the SR-IOV group using SR-IOV register enable/disable and NumVFs
> used as the number of the group size sounds simple from Spec and
> implementation POV.

Simple, however
- only works for SRIOV
- also limited, for example it's impossible to report an error

I would rather not decide now until we have a use-case.


> > 
> > 
> > > > 
> > > > 
> > > > > 
> > > > > > 
> > > > > > There's also the concern of VF migration under MRIOV where VFs might
> > > > > > not exit. I'm guessing
> > > > > > trying to include that right now is overthinking. So maybe
> > > > > > this should mention that VF Migration Capable should be clear.
> > > > > > 
> > > > > > 
> > > > > > I will add all this to the spec.
> > > > > > 
> > > > > > 
> > > > > > >        How about the converse?
> > > > > > > 
> > > > > > > 
> > > > > > >                    Each device within a group has a unique identifier. This identifier
> > > > > > >                    is the group member identifier.
> > > > > > > 
> > > > > > >                    Note: one can argue both ways whether the new device group handling
> > > > > > >                    functionality (this and following patches) is closer
> > > > > > >                    to a new device type or a new transport type.
> > > > > > > 
> > > > > > >                    However, I expect that we will add more features in the near future. To
> > > > > > >                    facilitate this as much as possible of the text is located in the new
> > > > > > >                    admin chapter.
> > > > > > > 
> > > > > > >                    I did my best to minimize transport-specific text.
> > > > > > > 
> > > > > > >                    Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> > > > > > >                    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > > >                    ---
> > > > > > >                     admin.tex   | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> > > > > > >                     content.tex |  2 ++
> > > > > > >                     2 files changed, 51 insertions(+)
> > > > > > >                     create mode 100644 admin.tex
> > > > > > > 
> > > > > > >                    diff --git a/admin.tex b/admin.tex
> > > > > > >                    new file mode 100644
> > > > > > >                    index 0000000..2bc7322
> > > > > > >                    --- /dev/null
> > > > > > >                    +++ b/admin.tex
> > > > > > >                    @@ -0,0 +1,49 @@
> > > > > > >                    +\section{Device groups}\label{sec:Basic Facilities of a Virtio Device / Device groups}
> > > > > > >                    +
> > > > > > >                    +It is occasionally useful to have a device control a group of
> > > > > > >                    +other devices. Terminology used in such cases:
> > > > > > >                    +
> > > > > > >                    +\begin{description}
> > > > > > >                    +\item[Device group]
> > > > > > >                    +        or just group, includes zero or more devices.
> > > > > > >                    +\item[Owner device]
> > > > > > >                    +        or owner, the device controlling the group.
> > > > > > >                    +\item[Member device]
> > > > > > >                    +        a device within a group. The owner device itself is not
> > > > > > >                    +       a member of the group.
> > > > > > >                    +\item[Member identifier]
> > > > > > >                    +        each member has this identifier, unique within the group
> > > > > > >                    +       and used to address it through the owner device.
> > > > > > >                    +\item[Group type identifier]
> > > > > > >                    +       specifies what kind of member devices there are in a
> > > > > > >                    +       group, how is the member identifier is interpreted
> > > > > > >                    +       and what kind of control the owner has.
> > > > > > >                    +       A given owner can control a single group of a given type,
> > > > > > >                    +       thus the type and the owner together identify the group.
> > > > > > >                    +       \footnote{Even though some group types only support
> > > > > > >                    +                       specific transports, group type identifiers
> > > > > > >                    +                       are global rather than transport-specific -
> > > > > > >                    +                       we don't expect a flood of new group types.}
> > > > > > >                    +\end{description}
> > > > > > >                    +
> > > > > > >                    +The following group types, and their identifiers, are currently specified):
> > > > > > >                    +\begin{description}
> > > > > > >                    +\item[SR-IOV group type (0x1)]
> > > > > > >                    +This device group has a PCI Single Root I/O Virtualization
> > > > > > >                    +(SR-IOV) physical function (PF) device as the owner and includes
> > > > > > >                    +all its SR-IOV virtual functions (VFs) as members (see
> > > > > > >                    +\hyperref[intro:PCIe]{[PCIe]}).
> > > > > > >                    +
> > > > > > >                    +The PF device itself is not a member of the group.
> > > > > > >                    +
> > > > > > >                    +The group type identifier for this group is 0x1.
> > > > > > >                    +
> > > > > > >                    +A member identifier for this group can have a value 0x1 to 0xFFFF
> > > > > > >                    +and equals the SR-IOV VF number of the member device (see
> > > > > > >                    +\hyperref[intro:PCIe]{[PCIe]}).
> > > > > > >                    +
> > > > > > >                    +Both owner and member devices for this group type use the Virtio
> > > > > > >                    +PCI transport (see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus}).
> > > > > > >                    +\end{description}
> > > > > > >                    +
> > > > > > >                    +
> > > > > > >                    diff --git a/content.tex b/content.tex
> > > > > > >                    index 0c2d917..ffe45c4 100644
> > > > > > >                    --- a/content.tex
> > > > > > >                    +++ b/content.tex
> > > > > > >                    @@ -491,6 +491,8 @@ \section{Exporting Objects}\label{sec:Basic Facilities of a Virtio Device / Expo
> > > > > > >                     types. It is RECOMMENDED that devices generate version 4
> > > > > > >                     UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.
> > > > > > > 
> > > > > > >                    +\input{admin.tex}
> > > > > > >                    +
> > > > > > >                     \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
> > > > > > > 
> > > > > > >                     We start with an overview of device initialization, then expand on the
> > > > > > > 
> > > > > > >                --
> > > > > > >                Tell her I'll be waiting, in the usual place.
> > > > > > > 
> > > > > > 
> > > > 
> >