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Re: [RFC v2 0/2] ui: Add a Wayland backend for Qemu UI (v2)

From: Daniel P . Berrangé
Subject: Re: [RFC v2 0/2] ui: Add a Wayland backend for Qemu UI (v2)
Date: Tue, 14 Sep 2021 17:29:49 +0100
User-agent: Mutt/2.0.7 (2021-05-04)

On Mon, Sep 13, 2021 at 03:20:34PM -0700, Vivek Kasireddy wrote:
> Why does Qemu need a new Wayland UI backend?
> The main reason why there needs to be a plain and simple Wayland backend
> for Qemu UI is to eliminate the Blit (aka GPU copy) that happens if using
> a toolkit like GTK or SDL (because they use EGL). The Blit can be eliminated
> by sharing the dmabuf fd -- associated with the Guest scanout buffer --
> directly with the Host compositor via the linux-dmabuf (unstable) protocol.
> Once properly integrated, it would be potentially possible to have the
> scanout buffer created by the Guest compositor be placed directly on a
> hardware plane on the Host thereby improving performance. Only Guest 
> compositors that use multiple back buffers (at-least 1 front and 1 back)
> and virtio-gpu would benefit from this work.

IME, QEMU already suffers from having too many barely maintained UI
implementations and iti s confusing to users. Using a toolkit like GTK
is generally a good thing, even if they don't enable the maximum
theoretical performance, because they reduce the long term maint burden.

I'm far from convinced that we should take on the maint of yet another
UI in QEMU, even if it does have some performance benefit, especially
if implemented using a very low level API like Wayland, that won't let
us easily add rich UI features.

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