Re: [PATCH v2 01/40] iommu: Introduce Shared Virtual Addressing API

["Christian König" <christian.koenig-5C7GfCeVMHo@public.gmane.org>]

Am 07.09.2018 um 17:45 schrieb Jean-Philippe Brucker:
> On 07/09/2018 09:55, Christian König wrote:
>> I will take this as an opportunity to summarize some of the requirements
>> we have for PASID management from the amdgpu driver point of view:
> That's incredibly useful, thanks :)
>
>> 1. We need to be able to allocate PASID between 1 and some maximum. Zero
>> is reserved as far as I know, but we don't necessary need a minimum.
> Should be fine. The PASID range is restricted by the PCI PASID
> capability, firmware description (for non-PCI devices), the IOMMU
> capacity, and what the device driver passes to iommu_sva_device_init.
> Not all IOMMUs reserve PASID 0 (AMD IOMMU without GIoSup doesn't, if I'm
> not mistaken), so the KFD driver will need to pass min_pasid=1 to make
> sure that 0 isn't allocated.
>
>> 2. We need to be able to allocate PASIDs without a process address space
>> backing it. E.g. our hardware uses PASIDs even without Shared Virtual
>> Addressing enabled to distinct clients from each other.
>>           Would be a pity if we need to still have a separate PASID
>> handling because the system wide is only available when IOMMU is turned on.
> I'm still not sure about this one. From my point of view we shouldn't
> add to the IOMMU subsystem helpers for devices without an IOMMU.

I agree on that.

> iommu-sva expects everywhere that the device has an iommu_domain, it's
> the first thing we check on entry. Bypassing all of this would call
> idr_alloc() directly, and wouldn't have any code in common with the
> current iommu-sva. So it seems like you need a layer on top of iommu-sva
> calling idr_alloc() when an IOMMU isn't present, but I don't think it
> should be in drivers/iommu/

In this case I question if the PASID handling should be under 
drivers/iommu at all.

See I can have a mix of VM context which are bound to processes (some 
few) and VM contexts which are standalone and doesn't care for a process 
address space. But for each VM context I need a distinct PASID for the 
hardware to work.

I can live if we say if IOMMU is completely disabled we use a simple ida 
to allocate them, but when IOMMU is enabled I certainly need a way to 
reserve a PASID without an associated process.

>> 3. Even after destruction of a process address space we need some grace
>> period before a PASID is reused because it can be that the specific
>> PASID is still in some hardware queues etc...
>>           At bare minimum all device drivers using process binding need
>> to explicitly note to the core when they are done with a PASID.
> Right, much of the horribleness in iommu-sva deals with this:
>
> The process dies, iommu-sva is notified and calls the mm_exit() function
> passed by the device driver to iommu_sva_device_init(). In mm_exit() the
> device driver needs to clear any reference to the PASID in hardware and
> in its own structures. When the device driver returns from mm_exit(), it
> effectively tells the core that it has finished using the PASID, and
> iommu-sva can reuse the PASID for another process. mm_exit() is allowed
> to block, so the device driver has time to clean up and flush the queues.
>
> If the device driver finishes using the PASID before the process exits,
> it just calls unbind().

Exactly that's what Michal Hocko is probably going to not like at all.

Can we have a different approach where each driver is informed by the 
mm_exit(), but needs to explicitly call unbind() before a PASID is reused?

During that teardown transition it would be ideal if that PASID only 
points to a dummy root page directory with only invalid entries.

>
>> 4. It would be nice to have to be able to set a "void *" for each
>> PASID/device combination while binding to a process which then can be
>> queried later on based on the PASID.
>>           E.g. when you have a per PASID/device structure around anyway,
>> just add an extra field.
> iommu_sva_bind_device() takes a "drvdata" pointer that is stored
> internally for the PASID/device combination (iommu_bond). It is passed
> to mm_exit(), but I haven't added anything for the device driver to
> query it back.

Nice! Looks like all we need additionally is a function to retrieve that 
based on the PASID.

>> 5. It would be nice to have to allocate multiple PASIDs for the same
>> process address space.
>>           E.g. some teams at AMD want to use a separate GPU address space
>> for their userspace client library. I'm still trying to avoid that, but
>> it is perfectly possible that we are going to need that.
> Two PASIDs pointing to the same process pgd? At first glance it seems
> feasible, maybe with a flag passed to bind() and a few changes to
> internal structures. It will duplicate ATC invalidation commands for
> each process address space change (munmap etc) so you might take a
> performance hit.
>
> Intel's SVM code has the SVM_FLAG_PRIVATE_PASID which seems similar to
> what you describe, but I don't plan to support it in this series (the
> io_mm model is already pretty complicated). I think it can be added
> without too much effort in a future series, though with a different flag
> name since we'd like to use "private PASID" for something else
> (https://www.spinics.net/lists/dri-devel/msg177007.html).

To be honest I hoped that you would say: No never! So that I have a good 
argument to pushback on such requirements :)

But if it's doable it would be at least nice to have for debugging.

Thanks a lot for working on that,
Christian.

>
> Thanks,
> Jean
>
>>           Additional to that it is sometimes quite useful for debugging
>> to isolate where exactly an incorrect access (segfault) is coming from.
>>
>> Let me know if there are some problems with that, especially I want to
>> know if there is pushback on #5 so that I can forward that :)
>>
>> Thanks,
>> Christian.
>>
>>> Thanks,
>>> Jean

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