virtio-comment

> From: Jason Wang <jasowang@redhat.com>
> Sent: Thursday, April 13, 2023 10:37 PM

> I'm not sure I get this. With VIRTIO_NET_F_LEGACY_HEADER, we don't need
> mediation for datapath. For the control path, mediation is a must for legacy and
> it's very easy to keep it work for modern, what's wrong with that?

There is virtio PCI VF.
The user who attaches this VF to the VM, it doesn’t know if its guest is to run legacy driver or 1.x
Hence hypervisor doesn’t want to run special stack when guest may (likely) be 1.x and device also supports 1.x.

Therefore, MMIO BAR 0 emulation is better solution in this use case without any mediation.
The current specification claim that transitional device "works", hence it is fine to extend BAR0 to be of MMIO type instead of IO type.

In some other cases control path or additional other mediation can be done.

On Fri, Apr 14, 2023 at 02:43:21AM +0000, Parav Pandit wrote:
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Thursday, April 13, 2023 10:37 PM
>
> > I'm not sure I get this. With VIRTIO_NET_F_LEGACY_HEADER, we don't need
> > mediation for datapath. For the control path, mediation is a must for legacy and
> > it's very easy to keep it work for modern, what's wrong with that?
>
> There is virtio PCI VF.
> The user who attaches this VF to the VM, it doesn’t know if its guest is to run legacy driver or 1.x
> Hence hypervisor doesn’t want to run special stack when guest may (likely) be 1.x and device also supports 1.x.
>
> Therefore, MMIO BAR 0 emulation is better solution in this use case without any mediation.
> The current specification claim that transitional device "works", hence it is fine to extend BAR0 to be of MMIO type instead of IO type.
>
> In some other cases control path or additional other mediation can be done.

Do you refer to the trick Jason proposed where BAR0 is memory but
otherwise matches legacy BAR0 exactly? Is this your preferred
solution at this point then?

--
MST

On Fri, Apr 14, 2023 at 02:43:21AM +0000, Parav Pandit wrote:
>
> > From: Jason Wang <jasowang@redhat.com>
> > Sent: Thursday, April 13, 2023 10:37 PM
>
> > I'm not sure I get this. With VIRTIO_NET_F_LEGACY_HEADER, we don't need
> > mediation for datapath. For the control path, mediation is a must for legacy and
> > it's very easy to keep it work for modern, what's wrong with that?
>
> There is virtio PCI VF.
> The user who attaches this VF to the VM, it doesn’t know if its guest is to run legacy driver or 1.x
> Hence hypervisor doesn’t want to run special stack when guest may (likely) be 1.x and device also supports 1.x.
>
> Therefore, MMIO BAR 0 emulation is better solution in this use case without any mediation.
> The current specification claim that transitional device "works", hence it is fine to extend BAR0 to be of MMIO type instead of IO type.
>
> In some other cases control path or additional other mediation can be done.

Do you refer to the trick Jason proposed where BAR0 is memory but
otherwise matches legacy BAR0 exactly? Is this your preferred
solution at this point then?

--
MST

> From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> open.org> On Behalf Of Michael S. Tsirkin
> Sent: Friday, April 14, 2023 2:57 AM

> Do you refer to the trick Jason proposed where BAR0 is memory but otherwise
> matches legacy BAR0 exactly? Is this your preferred solution at this point then?

We look at it again.
Above solution can work reliably only for a very small number of PF and that too with very special hardware circuitry due to the reset flow.

Therefore, for virtualization below interface is preferred.
a. For transitional device legacy configuration register transport over AQ,
Notification to utilize transitional device notification area of the BAR.

b. Non legacy interface of transitional and non-transitional PCI device to access direct PCI device without mediation.

> From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> open.org> On Behalf Of Michael S. Tsirkin
> Sent: Friday, April 14, 2023 2:57 AM

> Do you refer to the trick Jason proposed where BAR0 is memory but otherwise
> matches legacy BAR0 exactly? Is this your preferred solution at this point then?

We look at it again.
Above solution can work reliably only for a very small number of PF and that too with very special hardware circuitry due to the reset flow.

Therefore, for virtualization below interface is preferred.
a. For transitional device legacy configuration register transport over AQ,
Notification to utilize transitional device notification area of the BAR.

b. Non legacy interface of transitional and non-transitional PCI device to access direct PCI device without mediation.

On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> > open.org> On Behalf Of Michael S. Tsirkin
> > Sent: Friday, April 14, 2023 2:57 AM
>
> > Do you refer to the trick Jason proposed where BAR0 is memory but otherwise
> > matches legacy BAR0 exactly? Is this your preferred solution at this point then?
>
> We look at it again.
> Above solution can work reliably only for a very small number of PF and that too with very special hardware circuitry due to the reset flow.
>
> Therefore, for virtualization below interface is preferred.
> a. For transitional device legacy configuration register transport over AQ,

I don't get what this has to do with transitional ...

> Notification to utilize transitional device notification area of the BAR.

The vq transport does something like this, no?

> b. Non legacy interface of transitional and non-transitional PCI device to access direct PCI device without mediation.

So VF can either be accessed through AQ of PF, or through direct
mapping?

--
MST

On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-
> > open.org> On Behalf Of Michael S. Tsirkin
> > Sent: Friday, April 14, 2023 2:57 AM
>
> > Do you refer to the trick Jason proposed where BAR0 is memory but otherwise
> > matches legacy BAR0 exactly? Is this your preferred solution at this point then?
>
> We look at it again.
> Above solution can work reliably only for a very small number of PF and that too with very special hardware circuitry due to the reset flow.
>
> Therefore, for virtualization below interface is preferred.
> a. For transitional device legacy configuration register transport over AQ,

I don't get what this has to do with transitional ...

> Notification to utilize transitional device notification area of the BAR.

The vq transport does something like this, no?

> b. Non legacy interface of transitional and non-transitional PCI device to access direct PCI device without mediation.

So VF can either be accessed through AQ of PF, or through direct
mapping?

--
MST

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Sunday, April 16, 2023 4:44 PM
>
> On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > > From: virtio-comment@lists.oasis-open.org
> > > <virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
> > > Tsirkin
> > > Sent: Friday, April 14, 2023 2:57 AM
> >
> > > Do you refer to the trick Jason proposed where BAR0 is memory but
> > > otherwise matches legacy BAR0 exactly? Is this your preferred solution at
> this point then?
> >
> > We look at it again.
> > Above solution can work reliably only for a very small number of PF and that
> too with very special hardware circuitry due to the reset flow.
> >
> > Therefore, for virtualization below interface is preferred.
> > a. For transitional device legacy configuration register transport
> > over AQ,
>
> I don't get what this has to do with transitional ...
>
Typically, in current wordings, transitional is the device that supports legacy interface.
So, it doesn't have to be for the transitional.

I just wanted to highlight that a PCI VF device with its parent PCI PF device can transport the legacy interface commands.

> > Notification to utilize transitional device notification area of the BAR.
>
> The vq transport does something like this, no?
>
Notifications over a queuing interface unlikely can be a performant interface because one is configuration task and other is data path task.

> > b. Non legacy interface of transitional and non-transitional PCI device to
> access direct PCI device without mediation.
>
> So VF can either be accessed through AQ of PF, or through direct mapping?
Right. VF to access legacy registers using AQ of PF and continue non-legacy registers using direct mapping as done today.

On Tue, Apr 18, 2023 at 12:59?AM Parav Pandit <parav@nvidia.com> wrote:
>
>
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Sunday, April 16, 2023 4:44 PM
> >
> > On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > > > From: virtio-comment@lists.oasis-open.org
> > > > <virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
> > > > Tsirkin
> > > > Sent: Friday, April 14, 2023 2:57 AM
> > >
> > > > Do you refer to the trick Jason proposed where BAR0 is memory but
> > > > otherwise matches legacy BAR0 exactly? Is this your preferred solution at
> > this point then?
> > >
> > > We look at it again.
> > > Above solution can work reliably only for a very small number of PF and that
> > too with very special hardware circuitry due to the reset flow.
> > >
> > > Therefore, for virtualization below interface is preferred.
> > > a. For transitional device legacy configuration register transport
> > > over AQ,
> >
> > I don't get what this has to do with transitional ...
> >
> Typically, in current wordings, transitional is the device that supports legacy interface.
> So, it doesn't have to be for the transitional.
>
> I just wanted to highlight that a PCI VF device with its parent PCI PF device can transport the legacy interface commands.
>
> > > Notification to utilize transitional device notification area of the BAR.
> >
> > The vq transport does something like this, no?
> >
> Notifications over a queuing interface unlikely can be a performant interface because one is configuration task and other is data path task.

Note that current transport virtqueue only allows the notification via
MMIO. It introduces a command to get the address of the notification
area.

Thanks

>
> > > b. Non legacy interface of transitional and non-transitional PCI device to
> > access direct PCI device without mediation.
> >
> > So VF can either be accessed through AQ of PF, or through direct mapping?
> Right. VF to access legacy registers using AQ of PF and continue non-legacy registers using direct mapping as done today.
>

On Tue, Apr 18, 2023 at 12:59?AM Parav Pandit <parav@nvidia.com> wrote:
>
>
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Sunday, April 16, 2023 4:44 PM
> >
> > On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > > > From: virtio-comment@lists.oasis-open.org
> > > > <virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
> > > > Tsirkin
> > > > Sent: Friday, April 14, 2023 2:57 AM
> > >
> > > > Do you refer to the trick Jason proposed where BAR0 is memory but
> > > > otherwise matches legacy BAR0 exactly? Is this your preferred solution at
> > this point then?
> > >
> > > We look at it again.
> > > Above solution can work reliably only for a very small number of PF and that
> > too with very special hardware circuitry due to the reset flow.
> > >
> > > Therefore, for virtualization below interface is preferred.
> > > a. For transitional device legacy configuration register transport
> > > over AQ,
> >
> > I don't get what this has to do with transitional ...
> >
> Typically, in current wordings, transitional is the device that supports legacy interface.
> So, it doesn't have to be for the transitional.
>
> I just wanted to highlight that a PCI VF device with its parent PCI PF device can transport the legacy interface commands.
>
> > > Notification to utilize transitional device notification area of the BAR.
> >
> > The vq transport does something like this, no?
> >
> Notifications over a queuing interface unlikely can be a performant interface because one is configuration task and other is data path task.

Note that current transport virtqueue only allows the notification via
MMIO. It introduces a command to get the address of the notification
area.

Thanks

>
> > > b. Non legacy interface of transitional and non-transitional PCI device to
> > access direct PCI device without mediation.
> >
> > So VF can either be accessed through AQ of PF, or through direct mapping?
> Right. VF to access legacy registers using AQ of PF and continue non-legacy registers using direct mapping as done today.
>

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Sunday, April 16, 2023 4:44 PM
>
> On Sun, Apr 16, 2023 at 01:41:55PM +0000, Parav Pandit wrote:
> > > From: virtio-comment@lists.oasis-open.org
> > > <virtio-comment@lists.oasis- open.org> On Behalf Of Michael S.
> > > Tsirkin
> > > Sent: Friday, April 14, 2023 2:57 AM
> >
> > > Do you refer to the trick Jason proposed where BAR0 is memory but
> > > otherwise matches legacy BAR0 exactly? Is this your preferred solution at
> this point then?
> >
> > We look at it again.
> > Above solution can work reliably only for a very small number of PF and that
> too with very special hardware circuitry due to the reset flow.
> >
> > Therefore, for virtualization below interface is preferred.
> > a. For transitional device legacy configuration register transport
> > over AQ,
>
> I don't get what this has to do with transitional ...
>
Typically, in current wordings, transitional is the device that supports legacy interface.
So, it doesn't have to be for the transitional.

I just wanted to highlight that a PCI VF device with its parent PCI PF device can transport the legacy interface commands.

> > Notification to utilize transitional device notification area of the BAR.
>
> The vq transport does something like this, no?
>
Notifications over a queuing interface unlikely can be a performant interface because one is configuration task and other is data path task.

> > b. Non legacy interface of transitional and non-transitional PCI device to
> access direct PCI device without mediation.
>
> So VF can either be accessed through AQ of PF, or through direct mapping?
Right. VF to access legacy registers using AQ of PF and continue non-legacy registers using direct mapping as done today.