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Date: Sun, 9 Aug 2009 22:53:45 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] Re: virtio-blk performance and MSI
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To: Christoph Hellwig <hch@lst.de>
Cc: kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

On Sun, Aug 09, 2009 at 09:13:13PM +0200, Christoph Hellwig wrote:
> On Sun, Aug 09, 2009 at 08:49:23PM +0300, Avi Kivity wrote:
> > Sorry, confused.  Upstream doesn't use kvm for msi (everything's done in 
> > userspace).  What exactly blocks us here?
> 
> Hmm, good question.  Michael, do you know if we can just enabled
> MSI unconditionally in upstream qemu, or are there guests that can't
> cope with it?

Upstream or not, block should request MSI by default, the code in hw/msi
checks that host kernel can support it and only enables when it's safe.

And AFAIK all guests can cope with it but only if they are rebooted.
So we must have a flag to change the # of vectors, to have ability
to load images and migrate from state stored by old qemu.
And of course this option is also useful for troubleshooting.

-- 
MST