Re: [Qemu-devel] [RFC patch 0/6] vfio based pci pass-through for qemu/KVM on s390

[Alexander Graf <agraf@suse.de>]

On 22.09.14 22:47, Alex Williamson wrote:
> On Fri, 2014-09-19 at 13:54 +0200, frank.blaschka@de.ibm.com wrote:
>> This set of patches implements a vfio based solution for pci
>> pass-through on the s390 platform. The kernel stuff is pretty
>> much straight forward, but qemu needs more work.
>>
>> Most interesting patch is:
>>   vfio: make vfio run on s390 platform
>>
>> I hope Alex & Alex can give me some guidance how to do the changes
>> in an appropriate way. After creating a separate iommmu address space
>> for each attached PCI device I can successfully run the vfio type1
>> iommu. So If we could extend type1 not registering all guest memory
>> (see patch) I think we do not need a special vfio iommu for s390
>> for the moment.
>>
>> The patches implement the base pass-through support. s390 specific
>> virtualization functions are currently not included. This would
>> be a second step after the base support is done.
>>
>> kernel patches apply to linux-kvm-next
>>
>> KVM: s390: Enable PCI instructions
>> iommu: add iommu for s390 platform
>> vfio: make vfio build on s390
>>
>> qemu patches apply to qemu-master
>>
>> s390: Add PCI bus support
>> s390: implement pci instruction
>> vfio: make vfio run on s390 platform
>>
>> Thx for feedback and review comments
> 
> Sending patches as attachments makes it difficult to comment inline.
> 
> 2/6
>  - careful of the namespace as you're changing functions from static and
> exporting them
>  - doesn't seem like functions need to be exported, just non-static to
> call from s390-iommu.c
> 
> 6/6
>  - We shouldn't need to globally disable mmap, each VFIO region reports
> whether it supports mmap and vfio-pci on s390 should indicate mmap is
> not supported on the platform.

Can we emulate MMIO on mmap'ed regions by routing every memory access
via the kernel? It'd be slow, but at least make existing VFIO code
compatible.

>  - INTx should be done the same way, the interrupt index for INTx should
> report 0 count.  The current code likely doesn't handle this, but it
> should be easy to fix.
>  - s390_msix_notify() vs msix_notify() should be abstracted somewhere
> else.  How would an emulated PCI device with MSI-X support work?
>  - same for add_msi_route

Yes, please implement emulated PCI device support first, then do VFIO.


Alex