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

Baolu Lu
2024-09-27T02:12:54Z

On 9/27/24 4:03 AM, 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.
>
> 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?

It's per S1 nested domain in current VT-d design. It's simple but lacks
sharing of DID within a VM. We probably will change this later.

Thanks,
baolu