[PATCH v1] virtio-mem: minor clarification regarding read-access to unplugged blocks

[David Hildenbrand <david@redhat.com>]

Let's clarify that we don't expect all DMA to work with unplugged blocks.
We really only give guarantees when reading from unplugged memory blocks
via the CPU, e.g., as done by Linux when creating a system dump via
kdump: the new kernel will copy the content of the old (crashed) kernel
via the CPU to user space, from where it will find its final destination
inside the dump file. Note that dumping via makedumpfile under Linux will
avoid reading unplugged blocks completely.

This is a preparation for device passthrough to VMs, whereby such
dedicated devices might not be able to read from unplugged memory blocks.

Let's document that this scenario is possible, and why this handling is
in place at all.

Cc: teawater <teawaterz@linux.alibaba.com>
Cc: Marek Kedzierski <mkedzier@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 virtio-mem.tex | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/virtio-mem.tex b/virtio-mem.tex
index 85cfb61..62a1d02 100644
--- a/virtio-mem.tex
+++ b/virtio-mem.tex
@@ -246,8 +246,13 @@ \subsection{Device Operation}\label{sec:Device Types / Memory Device / Device Op
 
 The device MUST NOT change the content of plugged memory blocks.
 
-The device MUST allow to read from unplugged memory blocks inside
-the usable device-managed region.
+The device MUST allow the CPU to read from unplugged memory blocks inside
+the usable device-managed region. \footnote{To allow for simplified dumping of
+memory. The CPU is expected to copy such memory to another location before
+starting DMA.}
+
+The device MAY allow to read from unplugged memory blocks inside the
+usable device-managed region via DMA.
 
 The device MAY allow to read from unplugged memory blocks outside
 the usable device-managed region.
-- 
2.26.2