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Re: [Qemu-devel] [PATCH RFC 0/2] enable iommu with -device

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH RFC 0/2] enable iommu with -device
Date: Tue, 31 May 2016 09:44:22 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Mon, May 30, 2016 at 05:14:15PM +0300, Marcel Apfelbaum wrote:
> On 05/30/2016 04:43 PM, Peter Xu wrote:
> >On Mon, May 23, 2016 at 05:01:28PM +0300, Marcel Apfelbaum wrote:
> >>This is a proposal on how to create the iommu with
> >>'-device intel-iommu' instead of '-machine,iommu=on'.
> >>
> >>The device is part of the machine properties because we wanted
> >>to ensure it is created before any other PCI device.
> >>
> >>The alternative is to skip the bus_master_enable_region at
> >>the time the device is created. We can create this region
> >>at machine_done phase. (patch 1)
> >>
> >>Then we can enable sysbus devices for PC machines and make all the
> >>init steps inside the iommu realize function. (patch 2)
> >>
> >>The series is working, but a lot of issues are not resolved:
> >>   - minimum testing was done
> >>   - the iommu addr should be passed (maybe) in command line rather than 
> >> hard-coded
> >>   - enabling sysbus devices for PC machines is risky, I am not aware yet
> >>     of the side effects of this modification.
> >>   - I am not sure moving the bus_master_enable_region to machine_done
> >>     is with no undesired effects.
> >
> >I gave it a shot on the patches and it works nicely (of course no
> >complex configurations, like hot plug).
> >
> >Could you help introduce what will bring us if we use "-device" rather
> >than "-M" options?  Benefits I can see is that, we can specify
> >parameters with specific device, rather than messing them up in
> >"machine" options. Do we have any other benefits that I may have
> >missed?
> Hi Peter,
> Thanks for trying it!
> Mainly is about not hard-coding device options (e.g. PCI address for AMD 
> but also to avoid having devices added as a side-effect of some machine 
> option.
> This will bring as closer to a cleaner model of a modular machine.
> I plan to post a non-rfc version soon.

I just thought about whether we should support multiple IOMMUs in the
future (never know whether there would be a use case for
that). Anyway, looking forward to your non-rfc version. :)


-- peterx

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