Re: [PATCH v2 5/8] OvmfPkg/VirtioNetDxe: update TechNotes

[Laszlo Ersek <lersek@redhat.com>]

On 09/11/17 14:16, Brijesh Singh wrote:
> 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>
> ---
>  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 f39426fb13e4..c6ca341ead15 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
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>