linux-kernel - Re: [PATCH v6 0/2] Add p2p via dmabuf to habanalabs

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Date:   Tue, 14 Sep 2021 13:12:18 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <ogabbay@...nel.org>, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, christian.koenig@....com,
        galpress@...zon.com, sleybo@...zon.com,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, dledford@...hat.com,
        airlied@...il.com, alexander.deucher@....com, leonro@...dia.com,
        hch@....de, amd-gfx@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v6 0/2] Add p2p via dmabuf to habanalabs

On Tue, Sep 14, 2021 at 04:18:31PM +0200, Daniel Vetter wrote:
> On Sun, Sep 12, 2021 at 07:53:07PM +0300, Oded Gabbay wrote:
> > Hi,
> > Re-sending this patch-set following the release of our user-space TPC
> > compiler and runtime library.
> > 
> > I would appreciate a review on this.
> 
> I think the big open we have is the entire revoke discussions. Having the
> option to let dma-buf hang around which map to random local memory ranges,
> without clear ownership link and a way to kill it sounds bad to me.
> 
> I think there's a few options:
> - We require revoke support. But I've heard rdma really doesn't like that,
>   I guess because taking out an MR while holding the dma_resv_lock would
>   be an inversion, so can't be done. Jason, can you recap what exactly the
>   hold-up was again that makes this a no-go?

RDMA HW can't do revoke.

So we have to exclude almost all the HW and several interesting use
cases to enable a revoke operation.

>   - For non-revokable things like these dma-buf we'd keep a drm_master
>     reference around. This would prevent the next open to acquire
>     ownership rights, which at least prevents all the nasty potential
>     problems.

This is what I generally would expect, the DMABUF FD and its DMA
memory just floats about until the unrevokable user releases it, which
happens when the FD that is driving the import eventually gets closed.

I still don't think any of the complexity is needed, pinnable memory
is a thing in Linux, just account for it in mlocked and that is
enough.

Jason