OvmfPkg/VirtioNetDxe: update TechNotes

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>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>

1 files changed, 13 insertions, 7 deletions

diff --git a/OvmfPkg/VirtioNetDxe/TechNotes.txt b/OvmfPkg/VirtioNetDxe/TechNotes.txt
index fedbaee07e..40a22f66dc 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