Re: [Qemu-devel] [PATCH 00/13] IOMMU: Enable interrupt remapping for Intel IOMMU

[Radim Krčmář]

2016-02-19 12:43+0100, Jan Kiszka:
> On 2016-02-19 12:34, Peter Xu wrote:
>> On Fri, Feb 19, 2016 at 10:58:12AM +0100, Paolo Bonzini wrote:
>>>
>>> The source ID can be passed to the IOMMU using the MemTxAttrs mechanism.
>> 
>> I see that MemTxAttrs is not enabled yet? It's dropped in all the
>> accessors like memory_region_write_accessor()?
> 
> That would be a pity. How much work remains to fix that? Putting Peter
> Maydell on CC who apparently started this extension.

memory_region_dispatch_write() uses write_with_attrs, so I think we only
need to use one for MSI region and add attributes to direct writers
(like HPET and IOAPIC, because normal PCI devices should be passing
correct attributes).

> ...
>>> FWIW, Radim was thinking of interrupt remapping in the kvm-ioapic, which
>>> we have decided to set aside.
> 
>> Does it mean that we are planning not to support kernel_irqchip for
>> IR? Any quick reason?
> 
> The reason to split up the kernel irqchip is to reduce code complexity
> and, thus, attack surface of the kernel-side interface towards the
> guest. So, extending the "classic" in-kernel support with yet another
> feature is not really supporting that goal.

A fear of regressions after forcing people to switch from kernel ioapic
to qemu ioapic is the only argument for in-kernel support.
We'd like to use split irqchip even without IR.