[PATCH v2 3/3] shared memory: Define mmio registers

From: "Dr. David Alan Gilbert" <>

Define an MMIO interface to enumerate and map shared
memory regions.

Note: This hasn't been implemented.

Signed-off-by: Dr. David Alan Gilbert <>
---
 content.tex | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/content.tex b/content.tex
index 9838208..c3070bd 100644
--- a/content.tex
+++ b/content.tex
@@ -1726,6 +1726,28 @@ All register values are organized as Little Endian.
     selected by writing to \field{QueueSel}.
   }
   \hline 
+  \mmioreg{SHMId}{Shared memory id}{0x0ac}{W}{%
+    Writing to this register selects the shared memory region \ref{sec:Basic Facilities of a Virtio Device / Shared Memory Regions}
+    following operations on \field{SHMLenLow}, \field{SHMLenHigh},
+    \field{SHMBaseLow} and \field{SHMBaseHigh} apply to.
+  }
+  \hline 
+  \mmiodreg{SHMLenLow}{SHMLenHigh}{Shared memory region 64 bit long length}{0x0b0}{0x0xb4}{R}{%
+    These registers return the length of the shared memory
+    region, as defined by the device for the region selected by
+    the \field{SHMId} register.  Reading from a non-existent
+    region (i.e. where the ID written to \field{SHMId} is unused)
+    results in a length of -1.
+  }
+  \hline 
+  \mmiodreg{SHMBaseLow}{SHMBaseHigh}{Shared memory region 64 bit long physical address}{0x0b8}{0x0xbc}{W}{%
+    The driver writes these registers to indicate where it wishes
+    the device to map the shared memory region currently
+    selected.  The driver shall write \field{SHMBaseLow} first
+    and then \field{SHMBaseHigh}.  To avoid multiple remappings, the device shall not change
+    the mapping until it sees the write to \field{SHMBaseHigh}.
+  }
+  \hline 
   \mmioreg{ConfigGeneration}{Configuration atomicity value}{0x0fc}{R}{
     Reading from this register returns a value describing a version of the device-specific configuration space (see \field{Config}).
     The driver can then access the configuration space and, when finished, read \field{ConfigGeneration} again.
@@ -1761,6 +1783,9 @@ driver seeing an inconsistent configuration state.
 
 The device MUST NOT access virtual queue contents when \field{QueueReady} is zero (0x0).
 
+The device MUST NOT respond to SHM Base writes for none-existent
+shared memory regions.
+
 \drivernormative{\subsubsection}{MMIO Device Register Layout}{Virtio Transport Options / Virtio Over MMIO / MMIO Device Register Layout}
 The driver MUST NOT access memory locations not described in the
 table \ref{tab:Virtio Trasport Options / Virtio Over MMIO / MMIO Device Register Layout}
@@ -1815,6 +1840,13 @@ If both values are valid, it MUST read \field{DeviceID}
 and if its value is zero (0x0) MUST abort initialization and
 MUST NOT access any other register.
 
+Devices expecting shared memory regions MUST enumerate the
+regions by selecting each region in tern and checking that the
+length is not -1.
+
+Devices not expecting shared memory MUST NOT use the shared
+memory registers.
+
 Further initialization MUST follow the procedure described in
 \ref{sec:General Initialization And Device Operation / Device Initialization}~\nameref{sec:General Initialization And Device Operation / Device Initialization}.
 
-- 
2.20.1