Message-ID: <20200122165637.GD677983@stefanha-x1.localdomain>
Date:   Wed, 22 Jan 2020 16:56:37 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     "Liu, Jing2" <jing2.liu@...ux.intel.com>
Cc:     virtio-dev@...ts.oasis-open.org, slp@...hat.com,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Zha Bin <zhabin@...ux.alibaba.com>,
        Liu Jiang <gerry@...ux.alibaba.com>
Subject: Re: [virtio-dev][PATCH v1 1/2] virtio-mmio: Add MSI and different
 notification address support

On Mon, Jan 13, 2020 at 07:54:06PM +0800, Liu, Jing2 wrote:
> > > > >        \end{note}
> > > > >      }
> > > > >      \hline
> > > > > @@ -1671,25 +1671,23 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> > > > >        accesses apply to the queue selected by writing to \field{QueueSel}.
> > > > >      }
> > > > >      \hline
> > > > > -  \mmioreg{QueueNotify}{Queue notifier}{0x050}{W}{%
> > > > > -    Writing a value to this register notifies the device that
> > > > > -    there are new buffers to process in a queue.
> > > > > +  \mmioreg{QueueNotify}{Queue notifier}{0x050}{RW}{%
> > > > > +    Reading from the register returns the virtqueue notification configuration.
> > > > > -    When VIRTIO_F_NOTIFICATION_DATA has not been negotiated,
> > > > > -    the value written is the queue index.
> > > > > +    See \ref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Notification Address}
> > > > > +    for the configuration format.
> > > > > -    When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
> > > > > -    the \field{Notification data} value has the following format:
> > > > > +    Writing when the notification address calculated by the notification configuration
> > > > > +    is just located at this register.
> > > > I don't understand this sentence.  What happens when the driver writes
> > > > to this register?
> > > We're trying to define the notification mechanism that, driver MUST read
> > > 0x50 to get the notification configuration
> > > 
> > > and calculate the notify address. The writing case here is that, the notify
> > > address is just located here e.g. notify_base=0x50, notify_mul=0.
> > I still don't understand what this means.  It's just an English issue
> > and it will become clear if you can rephrase what you're saying.
> 
> Sure, let me try to explain it. :)
> 
> The different notification locations are calculated via the structure
> returned by reading this register.
> 
> le32 {
>     notify_base : 16;
>     notify_multiplier : 16;
> };
> 
> location=notify_base + queue_index * notify_multiplier
> 
> The location might be the same when mul=0, and furthermore, it might be
> equal to 0x50 (notify_base=0x50, notify_mul=0) so we make this register W
> too.
> 
> So we said, the register is RW and W is only for such scenario.
> 
> Feel free to tell me if it's still confusing.

I understand now:

  Devices that only require a single notify address may set
  notify_base=0x50 and notify_multiplier=0 to use the Queue Notifier
  register itself for notifications.  In this case the driver writes to
  Queue Notifier to notify the device that there are new buffers in a
  virtqueue.

Perhaps you could include this in the text.

> > > > > -    \lstinputlisting{notifications-le.c}
> > > > > -
> > > > > -    See \ref{sec:Virtqueues / Driver notifications}~\nameref{sec:Virtqueues / Driver notifications}
> > > > > -    for the definition of the components.
> > > > > +    See \ref{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Available Buffer Notifications}
> > > > > +    to see the notification data format.
> > > > >      }
> > > > >      \hline
> > > > >      \mmioreg{InterruptStatus}{Interrupt status}{0x60}{R}{%
> > > > >        Reading from this register returns a bit mask of events that
> > > > > -    caused the device interrupt to be asserted.
> > > > > +    caused the device interrupt to be asserted. This is only used
> > > > > +    when MSI is not enabled.
> > > > >        The following events are possible:
> > > > >        \begin{description}
> > > > >          \item[Used Buffer Notification] - bit 0 - the interrupt was asserted
> > > > > @@ -1703,7 +1701,7 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> > > > >      \mmioreg{InterruptACK}{Interrupt acknowledge}{0x064}{W}{%
> > > > >        Writing a value with bits set as defined in \field{InterruptStatus}
> > > > >        to this register notifies the device that events causing
> > > > > -    the interrupt have been handled.
> > > > > +    the interrupt have been handled. This is only used when MSI is not enabled.
> > > > >      }
> > > > >      \hline
> > > > >      \mmioreg{Status}{Device status}{0x070}{RW}{%
> > > > > @@ -1762,6 +1760,31 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
> > > > >        \field{SHMSel} is unused) results in a base address of
> > > > >        0xffffffffffffffff.
> > > > >      }
> > > > > +  \hline
> > > > > +  \mmioreg{MsiStatus}{MSI status}{0x0c0}{R}{%
> > > > > +    Reading from this register returns the global MSI enable/disable status and maximum
> > > > > +    number of virtqueues that device supports.
> > > > > +    \lstinputlisting{msi-status.c}
> > > > > +  }
> > > > Why is it necessary to combine the number of virtqueues and global
> > > > MSI enable/disable into a single 16-bit field?
> > > Originally, we want this 16-bit Read-Only, so we put some RO things together
> > > and separate
> > > 
> > > enable setting command to next register.
> > > 
> > > > virtio-mmio uses 32-bit registers.  It doesn't try hard to save register
> > > > space so it's strange to do it here (11-bit number of virtqueue field
> > > > but 32-bit QueueSel field).
> > > In order to improve performance/save register space,  we combine some data
> > > together.
> > > 
> > > For example, combine MSI cmd operator (e.g. enable/disable, vector setup)
> > > with argument (e.g. 1/0,  queue index).
> > > 
> > > But it seems we miss the consistency with QueueSel.  So do you think if the
> > > max queue number should be 32-bit,
> > > 
> > > which means it must be the same with QueueSel? If so, I guess we need some
> > > re-organization. :)
> > I suggest following the 32-bit register size convention unless there is
> > a specific reason why using other register sizes is absolutely necessary.
> 
> Yes, let's keep consistency with QueueSel and re-organize other registers.
> 
> I feel concern why Available Buffer Notifcations (section describing
> VIRTIO_F_NOTIFICATION_DATA) makes vq index as 16bit?

As you mentioned, the valid range of virtqueue numbers is only 16 bits
due to non-MMIO parts of the specification using 16 bits.

However, I think it makes sense to stick to the MMIO transport 32-bit
register size convention for consistency.  Devices just won't support
values above 0xffff.

Stefan

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