Re: [PATCHv2 1/3] iommu/io-pgtable: Add a quirk to use tlb_flush_all() for partial walk flush

[Robin Murphy <robin.murphy@arm.com>]

On 2021-06-22 15:27, Sai Prakash Ranjan wrote:
> Hi Robin,
> 
> On 2021-06-22 17:41, Robin Murphy wrote:
>> On 2021-06-22 08:11, Sai Prakash Ranjan wrote:
>>> Hi Robin,
>>>
>>> On 2021-06-21 21:15, Robin Murphy wrote:
>>>> On 2021-06-18 03:51, Sai Prakash Ranjan wrote:
>>>>> Add a quirk IO_PGTABLE_QUIRK_TLB_INV_ALL to invalidate entire context
>>>>> with tlb_flush_all() callback in partial walk flush to improve unmap
>>>>> performance on select few platforms where the cost of 
>>>>> over-invalidation
>>>>> is less than the unmap latency.
>>>>
>>>> I still think this doesn't belong anywhere near io-pgtable at all.
>>>> It's a driver-internal decision how exactly it implements a non-leaf
>>>> invalidation, and that may be more complex than a predetermined
>>>> boolean decision. For example, I've just realised for SMMUv3 we can't
>>>> invalidate multiple levels of table at once with a range command,
>>>> since if we assume the whole thing is mapped at worst-case page
>>>> granularity we may fail to invalidate any parts which are mapped as
>>>> intermediate-level blocks. If invalidating a 1GB region (with 4KB
>>>> granule) means having to fall back to 256K non-range commands, we may
>>>> not want to invalidate by VA then, even though doing so for a 2MB
>>>> region is still optimal.
>>>>
>>>> It's also quite feasible that drivers might want to do this for leaf
>>>> invalidations too - if you don't like issuing 512 commands to
>>>> invalidate 2MB, do you like issuing 511 commands to invalidate 2044KB?
>>>> - and at that point the logic really has to be in the driver anyway.
>>>>
>>>
>>> Ok I will move this to tlb_flush_walk() functions in the drivers. In 
>>> the previous
>>> v1 thread, you suggested to make the choice in iommu_get_dma_strict() 
>>> test,
>>> I assume you meant the test in iommu_dma_init_domain() with a flag or 
>>> was it
>>> the leaf driver(ex:arm-smmu.c) test of iommu_get_dma_strict() in 
>>> init_domain?
>>
>> Yes, I meant literally inside the same condition where we currently
>> set "pgtbl_cfg.quirks |= IO_PGTABLE_QUIRK_NON_STRICT;" in
>> arm_smmu_init_domain_context().
>>
> 
> Ok got it, thanks.
> 
>>> I am still a bit confused on where this flag would be? Should this be 
>>> a part
>>> of struct iommu_domain?
>>
>> Well, if you were to rewrite the config with an alternative set of
>> flush_ops at that point it would be implicit. For a flag, probably
>> either in arm_smmu_domain or arm_smmu_impl. Maybe a flag would be less
>> useful than generalising straight to a "maximum number of by-VA
>> invalidations it's worth sending individually" threshold value?
> 
> But then we would still need some flag to make this implementation
> specific (qcom specific for now) and this threshold would just be
> another condition although it would have been useful if this was
> generic enough.

Well, for that approach I assume we could do something like special-case 
0, or if it's a mutable per-domain value maybe just initialise it to 
SIZE_MAX or whatever such that it would never be reached in practice. 
Whichever way, it was meant to be implied that anything at the domain 
level would still be subject to final adjustment by the init_context hook.

Robin.

>> It's clear to me what overall shape and separation of responsibility is
>> most logical, but beyond that I don't have a particularly strong
>> opinion on the exact implementation; I've just been chucking ideas
>> around :)
>>
> 
> Your ideas are very informative and useful :)
> 
> Thanks,
> Sai
>