Re: [RFC] /dev/ioasid uAPI proposal

[Lu Baolu <baolu.lu@linux.intel.com>]

On 6/3/21 7:23 AM, Jason Gunthorpe wrote:
> On Wed, Jun 02, 2021 at 12:01:57PM +0800, Lu Baolu wrote:
>> On 6/2/21 1:26 AM, Jason Gunthorpe wrote:
>>> On Tue, Jun 01, 2021 at 07:09:21PM +0800, Lu Baolu wrote:
>>>
>>>> This version only covers 1) and 4). Do you think we need to support 2),
>>>> 3) and beyond?
>>>
>>> Yes aboslutely. The API should be flexable enough to specify the
>>> creation of all future page table formats we'd want to have and all HW
>>> specific details on those formats.
>>
>> OK, stay in the same line.
>>
>>>> If so, it seems that we need some in-kernel helpers and uAPIs to
>>>> support pre-installing a page table to IOASID.
>>>
>>> Not sure what this means..
>>
>> Sorry that I didn't make this clear.
>>
>> Let me bring back the page table types in my eyes.
>>
>>   1) IOMMU format page table (a.k.a. iommu_domain)
>>   2) user application CPU page table (SVA for example)
>>   3) KVM EPT (future option)
>>   4) VM guest managed page table (nesting mode)
>>
>> Each type of page table should be able to be associated with its IOASID.
>> We have BIND protocol for 4); We explicitly allocate an iommu_domain for
>> 1). But we don't have a clear definition for 2) 3) and others. I think
>> it's necessary to clearly define a time point and kAPI name between
>> IOASID_ALLOC and IOASID_ATTACH, so that other modules have the
>> opportunity to associate their page table with the allocated IOASID
>> before attaching the page table to the real IOMMU hardware.
> 
> In my mind these are all actions of creation..
> 
> #1 is ALLOC_IOASID 'to be compatible with thes devices attached to
>     this FD'
> #2 is ALLOC_IOASID_SVA
> #3 is some ALLOC_IOASID_KVM (and maybe the kvm fd has to issue this ioctl)
> #4 is ALLOC_IOASID_USER_PAGE_TABLE w/ user VA address or
>        ALLOC_IOASID_NESTED_PAGE_TABLE w/ IOVA address
> 
> Each allocation should have a set of operations that are allows
> map/unmap is only legal on #1. invalidate is only legal on #4, etc.

This sounds reasonable. The corresponding page table types and required
callbacks are also part of it.

> 
> How you want to split this up in the ioctl interface is a more
> interesting question. I generally like more calls than giant unwieldly
> multiplexer structs, but some things are naturally flags and optional
> modifications of a single ioctl.
> 
> In any event they should have a similar naming 'ALLOC_IOASID_XXX' and
> then a single 'DESTROY_IOASID' that works on all of them.
> 
>> I/O page fault handling is similar. The provider of the page table
>> should take the responsibility to handle the possible page faults.
> 
> For the faultable types, yes #3 and #4 should hook in the fault
> handler and deal with it.

Agreed.

Best regards,
baolu