linux-kernel - Re: [PULL] virtio

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Date:	Wed, 02 Nov 2011 06:52:44 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexey Kardashevskiy <aik@...abs.ru>,
	Amit Shah <amit.shah@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Krishna Kumar <krkumar2@...ibm.com>,
	Pawel Moll <pawel.moll@....com>,
	Wang Sheng-Hui <shhuiw@...il.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PULL] virtio

On Wed, 2011-11-02 at 11:39 +1030, Rusty Russell wrote:
> On Tue, 1 Nov 2011 14:42:23 +0200, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > On Tue, Nov 01, 2011 at 02:33:33PM +0200, Sasha Levin wrote:
> > > Unless Michael pointed this patch out, it would have broken (at least)
> > > the kvm tool in a non obvious way which would require a rather long
> > > session of 'git bisect' to figure out whats wrong.
> 
> I assumed not, until QEMU provided the second BAR it would simply fall
> back.  Or am I completely misunderstanding?

QEMU uses two bars within each virtio-pci device. First one (BAR 0) is
the PIO config BAR, and the second one (BAR 1) is the MSI-X BAR used for
the MSI-X table and the PBA.

Since the PCI spec allows you to have the MSI-X table and the PBA in
completely different regions pointed by two different BARs, we did so in
kvm tools and have BAR 1 pointing to the MSI-X table, and BAR 2 pointing
to the PBA ( - we *already* provided the second BAR and used it for
something else).

This means that with this patch, we'd load BAR 2 and use it as the
config space, instead of the PBA table which it really represents.

-- 

Sasha.

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