linux-kernel - Re: [Xen-devel] [RFC PATCH v2 2/9] hyper_dmabuf: architecture specification and reference guide

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Date:   Fri, 23 Feb 2018 16:15:00 +0000
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Dongwon Kim <dongwon.kim@...el.com>
CC:     <linux-kernel@...r.kernel.org>, <linaro-mm-sig@...ts.linaro.org>,
        <xen-devel@...ts.xenproject.org>, <sumit.semwal@...aro.org>,
        <dri-devel@...ts.freedesktop.org>, <mateuszx.potrola@...el.com>
Subject: Re: [Xen-devel] [RFC PATCH v2 2/9] hyper_dmabuf: architecture
 specification and reference guide

On Tue, Feb 13, 2018 at 05:50:01PM -0800, Dongwon Kim wrote:
> Reference document for hyper_DMABUF driver
> 
> Documentation/hyper-dmabuf-sharing.txt

This should likely be patch 1 in order for reviewers to have the
appropriate context.

> 
> Signed-off-by: Dongwon Kim <dongwon.kim@...el.com>
> ---
>  Documentation/hyper-dmabuf-sharing.txt | 734 +++++++++++++++++++++++++++++++++
>  1 file changed, 734 insertions(+)
>  create mode 100644 Documentation/hyper-dmabuf-sharing.txt
> 
> diff --git a/Documentation/hyper-dmabuf-sharing.txt b/Documentation/hyper-dmabuf-sharing.txt
> new file mode 100644
> index 000000000000..928e411931e3
> --- /dev/null
> +++ b/Documentation/hyper-dmabuf-sharing.txt
> @@ -0,0 +1,734 @@
> +Linux Hyper DMABUF Driver
> +
> +------------------------------------------------------------------------------
> +Section 1. Overview
> +------------------------------------------------------------------------------
> +
> +Hyper_DMABUF driver is a Linux device driver running on multiple Virtual
> +achines (VMs), which expands DMA-BUF sharing capability to the VM environment
> +where multiple different OS instances need to share same physical data without
> +data-copy across VMs.
> +
> +To share a DMA_BUF across VMs, an instance of the Hyper_DMABUF drv on the
> +exporting VM (so called, “exporter”) imports a local DMA_BUF from the original
> +producer of the buffer,

The usage of export and import in the above sentence makes it almost
impossible to understand.

> then re-exports it with an unique ID, hyper_dmabuf_id
> +for the buffer to the importing VM (so called, “importer”).

And this is even worse.

Maybe it would help to have some kind of flow diagram of all this
import/export operations, but please read below.

> +
> +Another instance of the Hyper_DMABUF driver on importer registers
> +a hyper_dmabuf_id together with reference information for the shared physical
> +pages associated with the DMA_BUF to its database when the export happens.
> +
> +The actual mapping of the DMA_BUF on the importer’s side is done by
> +the Hyper_DMABUF driver when user space issues the IOCTL command to access
> +the shared DMA_BUF. The Hyper_DMABUF driver works as both an importing and
> +exporting driver as is, that is, no special configuration is required.
> +Consequently, only a single module per VM is needed to enable cross-VM DMA_BUF
> +exchange.

IMHO I need a more generic view of the problem you are trying to solve
in the overview section. I've read the full overview, and I still have
no idea why you need all this.

I think the overview should contain at least:

1. A description of the problem you are trying to solve.
2. A high level description of the proposed solution.
3. How the proposed solution deals with the problem described in 1.

This overview is not useful for people that don't know which problem
you are trying to solve, like myself.

Thanks, Roger.