Re: [virtio-comment] [PATCH 5/5] virtio-pci: implement VIRTIO_F_QUEUE_STATE

[Jason Wang <jasowang@redhat.com>]

On Wed, Sep 6, 2023 at 4:33âPM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Sep 06, 2023 at 04:16:37PM +0800, Zhu Lingshan wrote:
> > This patch adds two new le16 fields to common configuration structure
> > to support VIRTIO_F_QUEUE_STATE in PCI transport layer.
> >
> > Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>
>
> I do not see why this would be pci specific at all.

This is the PCI interface for live migration. The facility is not
specific to PCI.

It can choose to reuse the common configuration or not, but the
semantic is general enough to be used by other transports. We can
introduce one for MMIO for sure.

>
> But besides I thought work on live migration will use
> admin queue. This was explicitly one of the motivators.

I think not. Using admin virtqueue will end up with several problems:

1) the feature is not self contained so at the end we need transport
specific facility to migrate the admin virtqueue
2) won't work in the nested environment, or we need complicated SR-IOV
emulation in order to work

>
> Poking at the device from the driver to migrate it
> is not going to work if the driver lives within guest.

This is by design to allow live migration to work in the nested layer.
And it's the way we've used for CPU and MMU. Anything may virtio
different here?

Thanks


>
>
>
>
> > ---
> >  transport-pci.tex | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/transport-pci.tex b/transport-pci.tex
> > index a5c6719..3161519 100644
> > --- a/transport-pci.tex
> > +++ b/transport-pci.tex
> > @@ -325,6 +325,10 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >          /* About the administration virtqueue. */
> >          le16 admin_queue_index;         /* read-only for driver */
> >          le16 admin_queue_num;         /* read-only for driver */
> > +
> > +     /* Virtqueue state */
> > +        le16 queue_avail_state;         /* read-write */
> > +        le16 queue_used_state;          /* read-write */
> >  };
> >  \end{lstlisting}
> >
> > @@ -428,6 +432,17 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >       The value 0 indicates no supported administration virtqueues.
> >       This field is valid only if VIRTIO_F_ADMIN_VQ has been
> >       negotiated.
> > +
> > +\item[\field{queue_avail_state}]
> > +        This field is valid only if VIRTIO_F_QUEUE_STATE has been
> > +        negotiated. The driver sets and gets the available state of
> > +        the virtqueue here (see \ref{sec:Virtqueues / Virtqueue State}).
> > +
> > +\item[\field{queue_used_state}]
> > +        This field is valid only if VIRTIO_F_QUEUE_STATE has been
> > +        negotiated. The driver sets and gets the used state of the
> > +        virtqueue here (see \ref{sec:Virtqueues / Virtqueue State}).
> > +
> >  \end{description}
> >
> >  \devicenormative{\paragraph}{Common configuration structure layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common configuration structure layout}
> > @@ -488,6 +503,9 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> >  present either a value of 0 or a power of 2 in
> >  \field{queue_size}.
> >
> > +If VIRTIO_F_QUEUE_STATE has not been negotiated, the device MUST ignore
> > +any accesses to \field{queue_avail_state} and \field{queue_used_state}.
> > +
> >  If VIRTIO_F_ADMIN_VQ has been negotiated, the value
> >  \field{admin_queue_index} MUST be equal to, or bigger than
> >  \field{num_queues}; also, \field{admin_queue_num} MUST be
> > --
> > 2.35.3
> >
> >
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