linux-kernel - Re: [PATCH v6 2/2] habanalabs: add support for dma-buf exporter

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Message-ID: <20211001145052.GO3544071@ziepe.ca>
Date:   Fri, 1 Oct 2021 11:50:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <ogabbay@...nel.org>
Cc:     Christian König <christian.koenig@....com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Gal Pressman <galpress@...zon.com>,
        Yossi Leybovich <sleybo@...zon.com>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        Dave Airlie <airlied@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Tomer Tayar <ttayar@...ana.ai>
Subject: Re: [PATCH v6 2/2] habanalabs: add support for dma-buf exporter

On Wed, Sep 29, 2021 at 12:17:35AM +0300, Oded Gabbay wrote:
> On Tue, Sep 28, 2021 at 8:36 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Sun, Sep 12, 2021 at 07:53:09PM +0300, Oded Gabbay wrote:
> > > From: Tomer Tayar <ttayar@...ana.ai>
> > >
> > > Implement the calls to the dma-buf kernel api to create a dma-buf
> > > object backed by FD.
> > >
> > > We block the option to mmap the DMA-BUF object because we don't support
> > > DIRECT_IO and implicit P2P.
> >
> > This statement doesn't make sense, you can mmap your dmabuf if you
> > like. All dmabuf mmaps are supposed to set the special bit/etc to
> > exclude them from get_user_pages() anyhow - and since this is BAR
> > memory not struct page memory this driver would be doing it anyhow.
> >
> But we block mmap the dmabuf fd from user-space.
> If you try to do it, you will get MAP_FAILED.

You do, I'm saying the above paragraph explaining *why* that was done
is not correct.

> > > We check the p2p distance using pci_p2pdma_distance_many() and refusing
> > > to map dmabuf in case the distance doesn't allow p2p.
> >
> > Does this actually allow the p2p transfer for your intended use cases?
>
> It depends on the system. If we are working bare-metal, then yes, it allows.
> If inside a VM, then no. The virtualized root complex is not
> white-listed and the kernel can't know the distance.
> But I remember you asked me to add this check, in v3 of the review IIRC.
> I don't mind removing this check if you don't object.

Yes, i tis the right code, I was curious how far along things have
gotten

> > Don't write to the kernel log from user space triggered actions
> at all ?

At all.

> It's the first time I hear about this limitation...

Oh? It is a security issue, we don't want to allow userspace to DOS
the kerne logging.

> How do you tell the user it has done something wrong ?

dev_dbg is the usual way and then users doing debugging can opt in to
the logging.


> > Why doesn't this return a sg_table * and an ERR_PTR?
> Basically I modeled this function after amdgpu_vram_mgr_alloc_sgt()
> And in that function they also return int and pass the sg_table as **
> 
> If it's critical I can change.

Please follow the normal kernel style

Jason