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Date: Tue, 26 Jul 2016 10:56:51 +0800
From: Peter Xu <peterx@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] x86: ioapic: upgrade emulated IOAPIC to
 ver 0x20
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-devel@nongnu.org, peter.maydell@linaro.org, pbonzini@redhat.com

On Tue, Jul 26, 2016 at 05:15:58AM +0300, Michael S. Tsirkin wrote:
> On Mon, Jul 25, 2016 at 07:36:24PM +0800, Peter Xu wrote:
> > IOMMU IR and IOAPIC legacy devices (e.g., e1000) cannot work well
> > together with some old Linux kernels (upstream before v4.0, or any
> > released RHEL kernels). This patch fixes it.
> > 
> > The problem is that: some old linux kernels (with IR enabled) only
> > support IOAPIC chips with version 0x20. New kernels after commit d32932d
> > ("x86/irq: Convert IOAPIC to use hierarchical irqdomain interfaces")
> > fixed this problem. To make sure we can work with even old kernels,
> > let's upgrade our IOAPIC to version 0x20.
> > 
> > This patch is only useful when vIOMMU IR is enabled (which still do not
> > support kernel IOAPIC). So here we are only upgrading QEMU IOAPIC chip
> > to version 0x20. For kernel based IOAPIC, the version will still be
> > 0x11.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> As a minimum, we need to make sure not to change this for
> old machine types.
> 
> Given the timing, limiting this for when IR is enabled
> would be a good idea I think.

Thanks Michael. Then let me cook another patch specified to IR, and
then we can choose either one we want.

-- peterx