Re: [virtio-comment] Re: [PATCH v1 3/8] device-context: Define the device context fields for device migration

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

On Wed, Nov 01, 2023 at 08:49:40AM +0000, Parav Pandit wrote:
> > So this:
> > 
> > +struct virtio_dev_ctx_pci_vq_cfg {
> > +        le16 vq_index;
> > +        le16 queue_size;
> > +        le16 queue_msix_vector;
> > +        le64 queue_desc;
> > +        le64 queue_driver;
> > +        le64 queue_device;
> > +};
> > 
> > 
> > duplicates a bunch of fields from this:
> > 
> Not really. Above is current VQ's configuration not visible in the config space directly.
> Below is already captured as part of VIRTIO_DEV_CTX_PCI_COMMON_CFG.

I really wanted to help you understand what I mean when I say
that the spec effort is duplicated.


> > struct virtio_pci_common_cfg {
> >         /* About the whole device. */
> >         __le32 device_feature_select;   /* read-write */
> >         __le32 device_feature;          /* read-only */
> >         __le32 guest_feature_select;    /* read-write */
> >         __le32 guest_feature;           /* read-write */
> >         __le16 msix_config;             /* read-write */
> >         __le16 num_queues;              /* read-only */
> >         __u8 device_status;             /* read-write */
> >         __u8 config_generation;         /* read-only */
> > 
> >         /* About a specific virtqueue. */
> >         __le16 queue_select;            /* read-write */
> >         __le16 queue_size;              /* read-write, power of 2. */
> >         __le16 queue_msix_vector;       /* read-write */
> >         __le16 queue_enable;            /* read-write */
> >         __le16 queue_notify_off;        /* read-only */
> >         __le32 queue_desc_lo;           /* read-write */
> >         __le32 queue_desc_hi;           /* read-write */
> >         __le32 queue_avail_lo;          /* read-write */
> >         __le32 queue_avail_hi;          /* read-write */
> >         __le32 queue_used_lo;           /* read-write */
> >         __le32 queue_used_hi;           /* read-write */
> > };
> > 
> > 
> > Except it's incomplete and I suspect that's actually a bug.
> > 
> It is not a bug.

For example. queue_enable is not there.

> There is some information duplicated. Above struct virtio_pci_common_cfg is the snapshot of registers being updated.
> While struct virtio_dev_ctx_pci_vq_cfg is capturing what is not visible in above config snapshot.
> i.e. queues which are already configured.
> For at that rare instance there is some duplication of some fields. but this is the exception part not be too much worried about.

Seems more of a rule than an exception - I don't see any fields that are
not also in config space.

> > 
> > Here's an idea: have a record per field. Use transport offsets as tags.
> > 
> There are just too many of them.
> They are logically clubbed in their native structures which are already defined.

Too many for what? If you don't like what I suggested find some way to
avoid duplicating everything please.


> > 
> > 
> > 
> > > > > > We need a stronger compatiblity story here I think.
> > > > > >
> > > > > > One way to show how it's designed to work would be to split the
> > > > > > patches. For example, add queue notify data and queue reset separately.
> > > > > I didn't follow the suggestion. Can you explain splitting patches
> > > > > and its relation
> > > > to the structure?
> > > Did I miss your response? Or it is in below msix?
> > 
> > exactly.
> > 
> > > > >
> > > > > >
> > > > > > Another is to add MSIX table migration option for when MSIX
> > > > > > table is passed through to guest.
> > > > > Yes, this will be added in future when there is actual hypervisor for it.
> > > >
> > > > You are tying the architecture to an extremely implementation specific
> > detail.
> > > >
> > > Which part?
> > >
> > > Not really. can you please which software will use MSI-X table migration?
> > 
> > Like, all and any? All hypervisors migrate the msi-x table.
> > 
> > > > Hypervisors *already* have migrate the MSIX table. Just in a
> > > > hypervisor specific way.
> > > MSI-X table is in the PCI BAR memory.
> > 
> > Exactly. And that means hypervisor should not read it from the device directly -
> > e.g. with an encrypted device it won't be able to.
> > 
> Yep.

And it follows device needs to include 

> > > > queue vector is an index into this table. So the index is migrated
> > > > through the device but the table itself has to be trapped and emulated by
> > hypervisor?
> > > Do you have a hypervisor and a platform that has not done MSI-X table
> > emulation for which you are asking to add?
> > > I don't know any.
> > > I am asking to add it when there is a _real_ user of it. Why it cannot be added
> > when the _real_ user arrive?
> > 
> > Real meaning actual hardware and software implementing it?  By this definition
> > there's no real user for migration in the spec at all - all of it can be done by
> > device specific means. 
> This is also an option if vendor specific commands are allowed.

Once you do that you can just throw all this spec effort out the window,
and do vdpa. I thought the point was to allow control plane in standard
hardware, so vendors can compete on best hardware and software is shared?

> > What's your point? By now we have a reasonably good
> > idea what hypervisors need to make migration portable. Let's either put all of it
> > in the spec or not bother at all.
> > 
> Maybe I was not clear.
> I am saying lets put the msix table when a user will find it useful in incremental manner.
> For example, lets say we put in the spec in 1.4 version Nov 23, will device implement it? mostly yes.
> Will existing software use it in 2024-25? mostly no, because most platform has complexity in this area as cpus have hard coded certain values.
> 
> So we are asking device to implement something that is not going to be used by any forcible future.
> 
> Hence, the request is, when the hypervisor/cpu vendor asks for it, it will be possible to add into the device.

If it is there then I expect hypervisors will use it, yes.

> > In other words, we need a bright line. I suggest a simple one: memory is
> > migrated by device, config space by hypervisor.  If not, suggest another one -
> > but it needs a reasonable rule based on a hardware not whatever software
> > found expedient to use.
> > 
> I really liked this bright line.
> Msix table like rest of the memory area of common config and device config will become part of this memory area.
> 
> I wish we also draw a good bright line between for near term vs long term to be practical.
>
> Adding MSI-X table in the spec is not hard now or in future, frankly.

Show me the patch.

-- 
MST


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