Re: [Qemu-devel] [PATCH V3] virtio: do not require IOMMU to be created in advance

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 09, 2017 at 11:05:36AM +0100, Paolo Bonzini wrote:
> 
> 
> On 09/03/2017 10:58, Jason Wang wrote:
> > 
> > 
> > On 2017年03月09日 17:28, Igor Mammedov wrote:
> >> On Thu, 9 Mar 2017 10:32:44 +0800
> >> Jason Wang<jasowang@redhat.com>  wrote:
> >>
> >>> On 2017年03月09日 00:40, Igor Mammedov wrote:
> >>>> On Tue, 7 Mar 2017 14:47:30 +0200
> >>>> Marcel Apfelbaum<marcel@redhat.com>   wrote:
> >>>>  
> >>>>> On 03/07/2017 11:09 AM, Jason Wang wrote:
> >>>>>> After commit 96a8821d2141 ("virtio: unbreak virtio-pci with IOMMU
> >>>>>> after caching ring translations"), IOMMU was required to be
> >>>>>> created in
> >>>>>> advance. This is because we can only get the correct dma_as after pci
> >>>>>> IOMMU (e.g intel_iommu) was initialized. This is suboptimal and
> >>>>>> inconvenient for user. This patch releases this by:
> >>>>>>
> >>>>>> - introduce a bus_master_ready method for PCIDeviceClass and trigger
> >>>>>>     this during pci_init_bus_master
> >>>>>> - implement virtio-pci method and 1) reset the dma_as 2) re-register
> >>>>>>     the memory listener to the new dma_as
> >> Instead of trying to fix up later it's possible to refuse
> >> adding iommu device if other devices has been added before
> >> it with -device/device_add.
> >> That would match current CLI semantics where device that
> >> others depend on should be listed on CLI before that others
> >> are listed.
> > 
> > Yes, but it works by chance in the past for the device that does not
> > want bus_master_as in their realize. This change may surprise their users.
> 
> There is another posssibility.  Create the address space at init time
> and add a container region instead of the bus_master_enable_region
> alias.  Then at machine_done time you create the bus_master_enable_region.
> 
> This removes the need for the callbacks and makes the MemoryListener
> just work.
> 
> Paolo

That's definitely cleaner than a callback, though a bit tricky
so needs a good comment explaining what is going on.
And then I think we can revert
96a8821d21411f10d77ea994af369c6e5c35a2cc, right?

-- 
MST