linux-kernel - Re: [virtio-dev] Re: [PATCH] Add virtio gpu driver.

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Date:	Thu, 26 Mar 2015 10:04:03 +0100
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	virtio-dev@...ts.oasis-open.org, Dave Airlie <airlied@...il.com>,
	Dave Airlie <airlied@...hat.com>,
	David Airlie <airlied@...ux.ie>,
	Rusty Russell <rusty@...tcorp.com.au>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
	"open list:VIRTIO CORE, NET..." 
	<virtualization@...ts.linux-foundation.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>
Subject: Re: [virtio-dev] Re: [PATCH] Add virtio gpu driver.

On Thu, Mar 26, 2015 at 09:42:47AM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > And is it possible to use offset within BAR and/or memory BARs?
> > If yes I'd strongly prefer this.
> 
> What is the point?  Do you want place virtio regions and vga framebuffer
> in the same pci bar?  Why?  virtio is mmio and traps into qemu on
> access, whereas the vga framebuffer is memory-backed (with dirty
> tracking turned on).  Don't think this is a good idea, even though the
> memory api would probably allow to do this.
> 
> cheers,
>   Gerd

Absolutely, it's pretty common to mix regions in a BAR.
For example, we have virtio kick (ioeventfd backed,
handled in kernel) in same BAR as common and device
specific configuration.

We did the same thing you are now doing with the
virtio BAR, and now we have to maintain two code
bases, virtio pci config was designed to be future proof
so why not use it?

This is mostly just making sure we don't paint ourselves into a corner.

-- 
MST
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