Re: [PATCH v4 06/17] PCI: add SIOV and IMS capability detection

[Thomas Gleixner]

On Mon, Nov 16 2020 at 14:02, Jason Gunthorpe wrote:
> On Mon, Nov 16, 2020 at 06:56:33PM +0100, Thomas Gleixner wrote:
>> On Mon, Nov 16 2020 at 11:46, Jason Gunthorpe wrote:
>> 
>> > On Mon, Nov 16, 2020 at 07:31:49AM +0000, Tian, Kevin wrote:
>> >
>> >> > The subdevices require PASID & IOMMU in native, but inside the guest there
>> >> > is no
>> >> > need for IOMMU unless you want to build SVM on top. subdevices work
>> >> > without
>> >> > any vIOMMU or hypercall in the guest. Only because they look like normal
>> >> > PCI devices we could map interrupts to legacy MSIx.
>> >> 
>> >> Guest managed subdevices on PF/VF requires vIOMMU. 
>> >
>> > Why? I've never heard we need vIOMMU for our existing SRIOV flows in
>> > VMs??
>> 
>> Handing PF/VF into the guest does not require it.
>> 
>> But if the PF/VF driver in the guest wants to create and manage the
>> magic mdev subdevices which require PASID support then you surely need
>> it.
>
> 'magic mdevs' are only one reason to use IMS in a guest. On mlx5 we
> might want to use IMS for VPDA devices. mlx5 can spawn a VDPA device
> in a guest, against a 'ADI', without ever requiring an IOMMU to do it.
>
> We don't even need IOMMU in the hypervisor to create the ADI, mlx5 has
> an internal secure IOMMU that can be used instead of the platform
> IOMMU.
>
> Not saying this is a major use case, or a reason not to link things to
> IOMMU detection, but lets be clear that a hard need for IOMMU is a
> another IDXD thing, not general.

Fair enough.

Thanks,

        tglx