Re: [virtio-dev] [PATCH v2 5/5] virtio-mmio: MSI vector and event mapping

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 21, 2020 at 09:54:33PM +0800, Jing Liu wrote:
> Bit 1 msi_sharing reported in the MsiState register indicates the mapping mode
> device uses.
> 
> Bit 1 is 0 - device uses MSI non-sharing mode. This indicates vector per event and
> fixed static vectors and events relationship. This fits for devices with a high interrupt
> rate and best performance;
> Bit 1 is 1 - device uses MSI sharing mode. This indicates vectors and events
> dynamic mapping and fits for devices not requiring a high interrupt rate.

It seems that sharing mode is a superset of non-sharing mode.
Isn't that right? E.g. with sharing mode drivers
can still avoid sharing if they like.

Maybe it should just be a hint to drivers whether to share
interrupts, instead of a completely different layout?

> Co-developed-by: Chao Peng <chao.p.peng@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> Co-developed-by: Liu Jiang <gerry@linux.alibaba.com>
> Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>
> Co-developed-by: Zha Bin <zhabin@linux.alibaba.com>
> Signed-off-by: Zha Bin <zhabin@linux.alibaba.com>
> Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
> ---
>  content.tex | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
>  msi-state.c |  3 ++-
>  2 files changed, 49 insertions(+), 2 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index dcf6c71..2fd1686 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -1770,7 +1770,8 @@ \subsection{MMIO Device Register Layout}\label{sec:Virtio Transport Options / Vi
>    \hline
>    \mmioreg{MsiState}{MSI state}{0x0c4}{R}{%
>      When VIRTIO_F_MMIO_MSI has been negotiated, reading
> -    from this register returns the global MSI enable/disable status.
> +    from this register returns the global MSI enable/disable status
> +    and whether device uses MSI sharing mode.
>      \lstinputlisting{msi-state.c}
>    }
>    \hline
> @@ -1926,12 +1927,18 @@ \subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virti
>  mask and unmask the MSI vector applying to the one selected by writing
>  to \field{MsiVecSel}.
>  
> +VIRTIO_MMIO_MSI_CMD_MAP_CONFIG command is to set the configuration event and MSI vector
> +mapping. VIRTIO_MMIO_MSI_CMD_MAP_QUEUE is to set the queue event and MSI vector
> +mapping. They SHOULD only be used in MSI sharing mode.
> +
>  \begin{lstlisting}
>  #define  VIRTIO_MMIO_MSI_CMD_ENABLE           0x1
>  #define  VIRTIO_MMIO_MSI_CMD_DISABLE          0x2
>  #define  VIRTIO_MMIO_MSI_CMD_CONFIGURE        0x3
>  #define  VIRTIO_MMIO_MSI_CMD_MASK             0x4
>  #define  VIRTIO_MMIO_MSI_CMD_UNMASK           0x5
> +#define  VIRTIO_MMIO_MSI_CMD_MAP_CONFIG       0x6
> +#define  VIRTIO_MMIO_MSI_CMD_MAP_QUEUE        0x7
>  \end{lstlisting}
>  
>  Setting a special NO_VECTOR value means disabling an interrupt for an event type.
> @@ -1941,10 +1948,49 @@ \subsubsection{Device Initialization}\label{sec:Virtio Transport Options / Virti
>  #define VIRTIO_MMIO_MSI_NO_VECTOR             0xffffffff
>  \end{lstlisting}
>  
> +\subparagraph{MSI Vector and Event Mapping}\label{sec:Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / Device Initialization / MSI Vector Configuration}
> +The reported \field{msi_sharing} bit in the \field{MsiState} return value shows
> +the MSI sharing mode that device uses.
> +
> +When \field{msi_sharing} bit is 0, it indicates the device uses non-sharing mode
> +and vector per event fixed static relationship is used. The first vector is for device
> +configuraiton change event, the second vector is for virtqueue 1, the third vector
> +is for virtqueue 2 and so on.
> +
> +When \field{msi_sharing} bit is 1, it indicates the device uses MSI sharing mode,
> +and the vector and event mapping is dynamic. Writing \field{MsiVecSel}
> +followed by writing VIRTIO_MMIO_MSI_CMD_MAP_CONFIG/VIRTIO_MMIO_MSI_CMD_MAP_QUEUE command
> +maps interrupts triggered by the configuration change/selected queue events respectively
> +to the corresponding MSI vector.
> +
> +\devicenormative{\subparagraph}{MSI Vector Configuration}{Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / MSI Vector Configuration}
> +
> +When the device reports \field{msi_sharing} bit as 0, it SHOULD support a number of
> +vectors that greater than the maximum number of virtqueues.
> +Device MUST report the number of vectors supported in \field{MsiVecNum}.
> +
> +When the device reports \field{msi_sharing} bit as 1, it SHOULD support at least
> +2 MSI vectors and MUST report in \field{MsiVecNum}. Device SHOULD support mapping any
> +event type to any vector under \field{MsiVecNum}.
> +
> +Device MUST support unmapping any event type (NO_VECTOR).
> +
> +The device SHOULD restrict the reported \field{msi_sharing} and \field{MsiVecNum}
> +to a value that might benefit system performance.
> +
> +\begin{note}
> +For example, a device which does not expect to send interrupts at a high rate might
> +return \field{msi_sharing} bit as 1.
> +\end{note}
> +
>  \drivernormative{\subparagraph}{MSI Vector Configuration}{Virtio Transport Options / Virtio Over MMIO / MMIO-specific Initialization And Device Operation / MSI Vector Configuration}
>  When VIRTIO_F_MMIO_MSI has been negotiated, driver should try to configure
>  and enable MSI.
>  
> +To set up the event and vector mapping for MSI sharing mode, driver SHOULD
> +write a valid \field{MsiVecSel} followed by VIRTIO_MMIO_MSI_CMD_MAP_CONFIG/VIRTIO_MMIO_MSI_CMD_MAP_QUEUE
> +command to map the configuration change/selected queue events respectively.
> +
>  To configure MSI vector, driver SHOULD firstly specify the MSI vector index by
>  writing to \field{MsiVecSel}.
>  Then notify the MSI address and data by writing to \field{MsiAddrLow}, \field{MsiAddrHigh},
> diff --git a/msi-state.c b/msi-state.c
> index b1fa0c1..d470be4 100644
> --- a/msi-state.c
> +++ b/msi-state.c
> @@ -1,4 +1,5 @@
>  le32 {
>      msi_enabled : 1;
> -    reserved : 31;
> +    msi_sharing: 1;
> +    reserved : 30;
>  };
> -- 
> 2.7.4
> 
> 
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