linux-kernel - Re: [PATCH v3 1/4] dma-buf: add support for virtio exported objects

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Message-ID: <20200514123007.GP206103@phenom.ffwll.local>
Date:   Thu, 14 May 2020 14:30:07 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     David Stevens <stevensd@...omium.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Tomasz Figa <tfiga@...omium.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO CORE, NET..." 
        <virtualization@...ts.linux-foundation.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, virtio-dev@...ts.oasis-open.org
Subject: Re: [PATCH v3 1/4] dma-buf: add support for virtio exported objects

On Thu, May 14, 2020 at 05:19:40PM +0900, David Stevens wrote:
> Sorry for the duplicate reply, didn't notice this until now.
> 
> > Just storing
> > the uuid should be doable (assuming this doesn't change during the
> > lifetime of the buffer), so no need for a callback.
> 
> Directly storing the uuid doesn't work that well because of
> synchronization issues. The uuid needs to be shared between multiple
> virtio devices with independent command streams, so to prevent races
> between importing and exporting, the exporting driver can't share the
> uuid with other drivers until it knows that the device has finished
> registering the uuid. That requires a round trip to and then back from
> the device. Using a callback allows the latency from that round trip
> registration to be hidden.

Uh, that means you actually do something and there's locking involved.
Makes stuff more complicated, invariant attributes are a lot easier
generally. Registering that uuid just always doesn't work, and blocking
when you're exporting?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch