[PATCH v4 5/8] OvmfPkg/VirtioNetDxe: update TechNotes

[Brijesh Singh <brijesh.singh@amd.com>]

In next patches we will update Virtio transmit to use the device-mapped
address of the caller-supplied packet. The patch documents the new model.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
---
 OvmfPkg/VirtioNetDxe/TechNotes.txt | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/OvmfPkg/VirtioNetDxe/TechNotes.txt b/OvmfPkg/VirtioNetDxe/TechNotes.txt
index fedbaee07ec7..40a22f66dc22 100644
--- a/OvmfPkg/VirtioNetDxe/TechNotes.txt
+++ b/OvmfPkg/VirtioNetDxe/TechNotes.txt
@@ -312,10 +312,14 @@ in the following:
   that is shared by all of the head descriptors. This virtio-net request header
   is never modified by the host.
 
-- Each tail descriptor is re-pointed to the caller-supplied packet buffer
-  whenever VirtioNetTransmit places the corresponding head descriptor on the
-  Available Ring. The caller is responsible to hang on to the unmodified buffer
-  until it is reported transmitted by VirtioNetGetStatus.
+- Each tail descriptor is re-pointed to the device-mapped address of the
+  caller-supplied packet buffer whenever VirtioNetTransmit places the
+  corresponding head descriptor on the Available Ring. A reverse mapping, from
+  the device-mapped address to the caller-supplied packet address, is saved in
+  an associative data structure that belongs to the driver instance.
+
+- Per spec, the caller is responsible to hang on to the unmodified packet
+  buffer until it is reported transmitted by VirtioNetGetStatus.
 
 Steps of packet transmission:
 
@@ -338,9 +342,11 @@ Steps of packet transmission:
 - Client code calls VirtioNetGetStatus. In case the Used Ring is empty, the
   function reports no Tx completion. Otherwise, a head descriptor's index is
   consumed from the Used Ring and recycled to the private stack. The client
-  code's original packet buffer address is fetched from the tail descriptor
-  (where it has been stored at VirtioNetTransmit time) and returned to the
-  caller.
+  code's original packet buffer address is calculated by fetching the
+  device-mapped address from the tail descriptor (where it has been stored at
+  VirtioNetTransmit time), and by looking up the device-mapped address in the
+  associative data structure. The reverse-mapped packet buffer address is
+  returned to the caller.
 
 - The Len field of the Used Ring Element is not checked. The host is assumed to
   have transmitted the entire packet -- VirtioNetTransmit had forced it below
-- 
2.9.5