[virtio-dev] Re: [PATCH 09/11] transport-pci: Describe PCI MMR dev config registers

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

On Fri, Mar 31, 2023 at 01:58:32AM +0300, Parav Pandit wrote:
> Legacy virtio configuration registers and adjacent
> device configuration registers are located somewhere
> in a memory BAR.
> 
> A new capability supplies the location of these registers
> which a driver can use to map I/O access to legacy
> memory mapped registers.
> 
> This gives the ability to locate legacy registers in either
> the existing memory BAR or as completely new BAR at BAR 0.
> 
> A below example diagram attempts to depicts it in an existing
> memory BAR.
> 
> +------------------------------+
> |Transitional                  |
> |MMR SRIOV VF                  |
> |                              |
> ++---------------+             |
> ||dev_id =       |             |
> ||{0x10f9-0x10ff}|             |
> |+---------------+             |
> |                              |
> ++--------------------+        |
> || PCIe ext cap = 0xB |        |
> || cfg_type = 10      |        |
> || offset   = 0x1000  |        |
> || bar      = A {0..5}|        |
> |+--|-----------------+        |
> |   |                          |
> |   |                          |
> |   |    +-------------------+ |
> |   |    | Memory BAR = A    | |
> |   |    |                   | |
> |   +------>+--------------+ | |
> |        |  |legacy virtio | | |
> |        |  |+ dev cfg     | | |
> |        |  |registers     | | |
> |        |  +--------------+ | |
> |        +-----------------+ | |
> +------------------------------+
> 
> Co-developed-by: Satananda Burla <sburla@marvell.com>
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> ---
>  transport-pci.tex | 33 +++++++++++++++++++++++++++++++--
>  1 file changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/transport-pci.tex b/transport-pci.tex
> index aeda4a1..55a6aa0 100644
> --- a/transport-pci.tex
> +++ b/transport-pci.tex
> @@ -168,6 +168,7 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
>  \item ISR Status
>  \item Device-specific configuration (optional)
>  \item PCI configuration access
> +\item Legacy memory mapped configuration registers (optional)
>  \end{itemize}
>  
>  Each structure can be mapped by a Base Address register (BAR) belonging to
> @@ -228,6 +229,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
>  #define VIRTIO_PCI_CAP_SHARED_MEMORY_CFG 8
>  /* Vendor-specific data */
>  #define VIRTIO_PCI_CAP_VENDOR_CFG        9
> +/* Legacy configuration registers capability */
> +#define VIRTIO_PCI_CAP_LEGACY_MMR_CFG    10
>  \end{lstlisting}
>  
>          Any other value is reserved for future use.
> @@ -682,6 +685,18 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>  Configuration Space / Legacy Interface: Device Configuration
>  Space}~\nameref{sec:Basic Facilities of a Virtio Device / Device Configuration Space / Legacy Interface: Device Configuration Space} for workarounds.
>  
> +\paragraph{Transitional MMR Interface: A Note on Configuration Registers}
> +\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities / Common configuration structure layout / Transitional MMR Interface: A Note on Configuration Registers}
> +
> +The transitional MMR device MUST present legacy virtio registers
> +consisting of legacy common configuration registers followed by
> +legacy device specific configuration registers described in section
> +\ref{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities / Common configuration structure layout / Legacy Interfaces: A Note on Configuration Registers}
> +in a memory region PCI BAR.


So considering common legacy registers. How exactly is INT#x
handled? It's required for old guests since they fail over to that
when MSI-X fails for some reason.




> +
> +The transitional MMR device MUST provide the location of the
> +legacy virtio configuration registers using a legacy memory mapped
> +registers capability described in section \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities / Transitional MMR Interface: Legacy Memory Mapped Configuration Registers Capability}.
>  
>  \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
>  
> @@ -956,9 +971,23 @@ \subsubsection{PCI configuration access capability}\label{sec:Virtio Transport O
>  specified by some other Virtio Structure PCI Capability
>  of type other than \field{VIRTIO_PCI_CAP_PCI_CFG}.
>  
> +\subsubsection{Transitional MMR Interface: Legacy Memory Mapped Configuration Registers Capability}
> +\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities / Transitional MMR Interface: Legacy Memory Mapped Configuration Registers Capability}
> +
> +The optional VIRTIO_PCI_CAP_LEGACY_MMR_CFG capability defines
> +the location of the legacy virtio configuration registers
> +followed by legacy device specific configuration registers in
> +the memory region BAR for the transitional MMR device.
> +
> +The \field{cap.offset} MUST be 4-byte aligned.
> +The \field{cap.offset} SHOULD be 4KBytes aligned and
> +\field{cap.length} SHOULD be 4KBytes.
> +
> +The transitional MMR device MUST present a legacy configuration
> +memory mapped registers capability using \field{virtio_pcie_ext_cap}.
> +
>  \subsubsection{Legacy Interface: A Note on Feature Bits}
> -\label{sec:Virtio Transport Options / Virtio Over PCI Bus /
> -Virtio Structure PCI Capabilities / Legacy Interface: A Note on Feature Bits}
> +\label{sec:Virtio Transport Options / Virtio Over PCI Bus / Virtio Structure PCI Capabilities / Legacy Interface: A Note on Feature Bits}
>  
>  Only Feature Bits 0 to 31 are accessible through the
>  Legacy Interface. When used through the Legacy Interface,
> -- 
> 2.26.2


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