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Subject: RE: [PATCH 10/11] transport-pci: Use driver notification PCI capability
> From: Michael S. Tsirkin <mst@redhat.com> > Sent: Wednesday, April 12, 2023 12:43 AM > To: Parav Pandit <parav@nvidia.com> > Cc: virtio-dev@lists.oasis-open.org; cohuck@redhat.com; virtio- > comment@lists.oasis-open.org; Shahaf Shuler <shahafs@nvidia.com>; > Satananda Burla <sburla@marvell.com> > Subject: Re: [PATCH 10/11] transport-pci: Use driver notification PCI capability > > On Wed, Apr 12, 2023 at 04:37:05AM +0000, Parav Pandit wrote: > > > > > > > From: Michael S. Tsirkin <mst@redhat.com> > > > Sent: Wednesday, April 12, 2023 12:31 AM > > > > > > On Fri, Mar 31, 2023 at 01:58:33AM +0300, Parav Pandit wrote: > > > > PCI devices support memory BAR regions for performant driver > > > > notifications using the notification capability. > > > > Enable transitional MMR devices to use it in simpler manner. > > > > > > > > Co-developed-by: Satananda Burla <sburla@marvell.com> > > > > Signed-off-by: Parav Pandit <parav@nvidia.com> > > > > --- > > > > transport-pci.tex | 28 ++++++++++++++++++++++++++++ > > > > 1 file changed, 28 insertions(+) > > > > > > > > diff --git a/transport-pci.tex b/transport-pci.tex index > > > > 55a6aa0..4fd9898 100644 > > > > --- a/transport-pci.tex > > > > +++ b/transport-pci.tex > > > > @@ -763,6 +763,34 @@ \subsubsection{Notification structure > > > > layout}\label{sec:Virtio Transport Options cap.length >= > > > > queue_notify_off * notify_off_multiplier + 4 \end{lstlisting} > > > > > > > > +\paragraph{Transitional MMR Interface: A note on Notification > > > > +Capability} \label{sec:Virtio Transport Options / Virtio Over PCI > > > > +Bus / Virtio Structure PCI Capabilities / Notification capability > > > > +/ Transitional MMR Interface} > > > > + > > > > +The transitional MMR device benefits from receiving driver > > > > +notifications at the Queue Notification address offered using the > > > > +notification capability, rather than via the memory mapped legacy > > > > +QueueNotify configuration register. > > > > + > > > > +Transitional MMR device uses same Queue Notification address > > > > +within a BAR for all virtqueues: > > > > +\begin{lstlisting} > > > > +cap.offset > > > > +\end{lstlisting} > > > > + > > > > +The transitional MMR device MUST support Queue Notification > > > > +address within a BAR for all virtqueues at: > > > > +\begin{lstlisting} > > > > +cap.offset > > > > +\end{lstlisting} > > > > + > > > > +The transitional MMR driver that wants to use driver > > > > +notifications offered using notification capability MUST use same > > > > +Queue Notification address within a BAR for all virtqueues at: > > > > + > > > > +\begin{lstlisting} > > > > +cap.offset > > > > +\end{lstlisting} > > > > + > > > Why? What exactly is going on here? legacy drivers will not do this. > > > > Legacy driver does in the q notify register that was sandwitched in between > of slow configuration registers. > > This is the notification offset for the hypervisor driver to perform the > notification on behalf of the guest driver so that the acceleration available for > the non-transitional device can be utilized here as well. > > I don't get it. What acceleration? for guests you need a separate page so card > can be mapped directly while config causes an exit. But hypervisor can access > any register without vmexits. Typically when guest VM writes to IOBAR q notification register, a vmexit occurs. On that occurrence, hypervisor driver forwards the q notification using the q notification region which is defined by struct virtio_pci_notify_cap.
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