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Re: [Qemu-devel] [PATCH for-2.10 0/5] x86/vIOMMU: move init back to mach

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH for-2.10 0/5] x86/vIOMMU: move init back to machine init phase
Date: Tue, 11 Apr 2017 11:22:38 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)

Peter Xu <address@hidden> writes:

> On Fri, Mar 31, 2017 at 07:17:34PM +0300, Michael S. Tsirkin wrote:
>> On Fri, Mar 31, 2017 at 03:36:28PM +0800, Peter Xu wrote:
>> > At the very beginning, the x86 vIOMMUs are created via "-M iommu=on".
>> > We moved one step further a year ago to have the vIOMMUs just like a
>> > general device, so that we can init them with far more specific
>> > parameters with "-device" interface.
>> > 
>> > However, gradually we found that problem starts to occur due to this.
>> > The main issue is that the DMA address space of any PCI device is
>> > postponed to be init after device realization, while some devices'
>> > realizations would depend on this address space. That looks like a
>> > dead lock. We have patches and solutions for different single problem,
>> > but, maybe it's time we can consider to solve the root cause this
>> > time, of course after 2.9 release.
>> > 
>> > This series tries to solve the root cause, and move vIOMMU inits back
>> > to machine init for x86 platforms. Then, we'll have solid DMA address
>> > space for each device even during realization.
>> > 
>> > Please kindly review. Thanks.
>> I think it's a clean way to do it at a high level.
>> However I would like to set a tag in the class
>> rather than listing specific devices.
>> Also, init order should be consistent for all machines
>> not just q35.
> I agree that we may finally need a tag if we want to have a general
> solution for device init ordering. However I was thinking maybe for
> x86 IOMMUs we should even move the init earlier than an ordered device
> list, considering the integrated devices are created during machine
> init, and it's before the general device init loop.
> Actually, iiuc that's also following how the real hardware works -
> since the IOMMU unit (now we only have the root vIOMMU) belongs to
> north bridge, so imho in emulation codes we'd better follow how the
> hardware works if possible (I believe in hardware IOMMUs should be
> inited along with north bridge).
> (btw, maybe I should mark at least the first patch as RFC...)

No objection to fixing yet another initialization order problem by
making it yet another special case, but I think we should really, really
sit down and try to come up with a *generic* way to express
initialization order constraints.  This is a modelling problem: we need
to model how physical hardware resets.  As long as we don't, we're
damned to tinker with this rickety tower of special cases.

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