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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. -- MST
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