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