Re: [PATCH v2 00/19] iommufd: Add VIOMMU infrastructure (Part-1)

[Yi Liu <yi.l.liu@intel.com>]

On 2024/9/27 04:03, Nicolin Chen wrote:
> On Thu, Sep 26, 2024 at 04:47:02PM +0800, Yi Liu wrote:
>> On 2024/9/26 02:55, Nicolin Chen wrote:
>>> On Wed, Sep 25, 2024 at 06:30:20PM +0800, Yi Liu wrote:
>>>> Hi Nic,
>>>>
>>>> On 2024/8/28 00:59, Nicolin Chen wrote:
>>>>> This series introduces a new VIOMMU infrastructure and related ioctls.
>>>>>
>>>>> IOMMUFD has been using the HWPT infrastructure for all cases, including a
>>>>> nested IO page table support. Yet, there're limitations for an HWPT-based
>>>>> structure to support some advanced HW-accelerated features, such as CMDQV
>>>>> on NVIDIA Grace, and HW-accelerated vIOMMU on AMD. Even for a multi-IOMMU
>>>>> environment, it is not straightforward for nested HWPTs to share the same
>>>>> parent HWPT (stage-2 IO pagetable), with the HWPT infrastructure alone.
>>>>
>>>> could you elaborate a bit for the last sentence in the above paragraph?
>>>
>>> Stage-2 HWPT/domain on ARM holds a VMID. If we share the parent
>>> domain across IOMMU instances, we'd have to make sure that VMID
>>> is available on all IOMMU instances. There comes the limitation
>>> and potential resource starving, so not ideal.
>>
>> got it.
>>
>>> Baolu told me that Intel may have the same: different domain IDs
>>> on different IOMMUs; multiple IOMMU instances on one chip:
>>> https://lore.kernel.org/linux-iommu/cf4fe15c-8bcb-4132-a1fd-b2c8ddf2731b@linux.intel.com/
>>> So, I think we are having the same situation here.
>>
>> yes, it's called iommu unit or dmar. A typical Intel server can have
>> multiple iommu units. But like Baolu mentioned in that thread, the intel
>> iommu driver maintains separate domain ID spaces for iommu units, which
>> means a given iommu domain has different DIDs when associated with
>> different iommu units. So intel side is not suffering from this so far.
> 
> An ARM SMMU has its own VMID pool as well. The suffering comes
> from associating VMIDs to one shared parent S2 domain.

Is this because of the VMID is tied with a S2 domain?

> Does a DID per S1 nested domain or parent S2? If it is per S2,
> I think the same suffering applies when we share the S2 across
> IOMMU instances?

per S1 I think. The iotlb efficiency is low as S2 caches would be
tagged with different DIDs even the page table is the same. :)

>>> Adding another vIOMMU wrapper on the other hand can allow us to
>>> allocate different VMIDs/DIDs for different IOMMUs.
>>
>> that looks like to generalize the association of the iommu domain and the
>> iommu units?
> 
> A vIOMMU is a presentation/object of a physical IOMMU instance
> in a VM.

a slice of a physical IOMMU. is it? and you treat S2 hwpt as a resource
of the physical IOMMU as well.

> This presentation gives a VMM some capability to take
> advantage of some of HW resource of the physical IOMMU:
> - a VMID is a small HW reousrce to tag the cache;
> - a vIOMMU invalidation allows to access device cache that's
>    not straightforwardly done via an S1 HWPT invalidation;
> - a virtual device presentation of a physical device in a VM,
>    related to the vIOMMU in the VM, which contains some VM-level
>    info: virtual device ID, security level (ARM CCA), and etc;
> - Non-PRI IRQ forwarding to the guest VM;
> - HW-accelerated virtualization resource: vCMDQ, AMD VIOMMU;

might be helpful to draw a diagram to show what the vIOMMU obj contains.:)

-- 
Regards,
Yi Liu