linux-kernel - Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 23 Jun 2021 15:50:45 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     Christian König <ckoenig.leichtzumerken@...il.com>,
        Christian König <christian.koenig@....com>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Doug Ledford <dledford@...hat.com>,
        Tomer Tayar <ttayar@...ana.ai>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export
 FD for DMA-BUF

On Wed, Jun 23, 2021 at 09:43:04PM +0300, Oded Gabbay wrote:

> Can you please explain why it is so important to (allow) access them
> through the CPU ?

It is not so much important, as it reflects significant design choices
that are already tightly baked into alot of our stacks. 

A SGL is CPU accessible by design - that is baked into this thing and
places all over the place assume it. Even in RDMA we have
RXE/SWI/HFI1/qib that might want to use the CPU side (grep for sg_page
to see)

So, the thing at the top of the stack - in this case the gaudi driver
- simply can't assume what the rest of the stack is going to do and
omit the CPU side. It breaks everything.

Logan's patch series is the most fully developed way out of this
predicament so far.

> The whole purpose is that the other device accesses my device,
> bypassing the CPU.

Sure, but you don't know that will happen, or if it is even possible
in any given system configuration. The purpose is to allow for that
optimization when possible, not exclude CPU based approaches.

Jason