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

[Peter Xu <peterx@redhat.com>]

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?

Thanks!

-- peterx