[PATCH v2 2/3] transport-pci: introduce page_shift field for SHM

[Sergio Lopez <slp@redhat.com>]

Borrowing 8 bits from the 16 bit padding in virtio_pci_cap to hold
a page_shift field which is used to derive the page size supported
by the device.

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 transport-pci.tex | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/transport-pci.tex b/transport-pci.tex
index a5c6719..efb174a 100644
--- a/transport-pci.tex
+++ b/transport-pci.tex
@@ -147,7 +147,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
         u8 cfg_type;    /* Identifies the structure. */
         u8 bar;         /* Where to find it. */
         u8 id;          /* Multiple capabilities of the same type */
-        u8 padding[2];  /* Pad to full dword. */
+        u8 page_shift;  /* Page shift for this shared memory region. */
+        u8 padding[1];  /* Pad to full dword. */
         le32 offset;    /* Offset within bar. */
         le32 length;    /* Length of the structure, in bytes. */
 };
@@ -222,6 +223,13 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
         of a certain type. If the device type does not specify the meaning of
         this field, its contents are undefined.
 
+\item[\field{page_shift}]
+        If \field{cfg_type} is set to VIRTIO_PCI_CAP_SHARED_MEMORY_CFG, the
+        device MUST provide the page shift derived from the supported page size
+        for this shared memory region. The page shift is derived from the page
+        size by using this formula:
+        $page size = 1 << (page shift + 12)$.
+        See also \ref {sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Page alignment restrictions}.
 
 \item[\field{offset}]
         indicates where the structure begins relative to the base address associated
-- 
2.49.0