linux-kernel - Re: [PATCH] virtio-pci: Use PCI MMIO instead of PIO when available

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Date:	Thu, 13 Oct 2011 10:22:36 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Alexey Kardashevskiy <aik@...abs.ru>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio-pci: Use PCI MMIO instead of PIO when available

On Wed, 12 Oct 2011 14:39:06 +1100, Alexey Kardashevskiy <aik@...abs.ru> wrote:
> Currently virtio-pci is specced so that configuration of the device is
> done through a PCI IO space (via BAR 0 of the virtual PCI device).
> However, use of PCI IO space (aka PIO) is long deprecated, and can be
> awkward to use on some systems (for example IBM pSeries machines
> typically have many PCI domains, and not all firmware/hypervisor
> versions necessarily support PCI PIO access on all domains).
> 
> Therefore, it would be preferable for the virtio virtual PCI device to
> advertise a PCI memory space (aka MMIO) BAR and have configuration
> done through this interface instead.  This can be done backwards
> compatibly by advertising the MMIO BAR in addition to the existing PIO
> BAR so that the guest driver can choose whichever interface.
> 
> In anticipation of adding such an MMIO BAR to virtio host-side
> implementations (e.g. qemu), this patch updates the Linux virtio-pci
> driver to attempt to use BAR 2 (which will be MMIO) in preference to
> the existing PIO BAR 0.

Applied.  Had to convert spaces to tabs though...

Thanks,
Rusty.
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