Re: tlbi va, vaa vs. val, vaal

[Marc Zyngier <marc.zyngier@arm.com>]

On 27/02/15 10:33, Will Deacon wrote:
> On Fri, Feb 27, 2015 at 10:29:06AM +0000, Marc Zyngier wrote:
>> On 27/02/15 10:24, Will Deacon wrote:
>>> On Fri, Feb 27, 2015 at 12:12:32AM +0000, Mario Smarduch wrote:
>>>> I noticed kernel tlbflush.h use tlbi va*, vaa* variants instead of
>>>> val, vaal ones. Reading the manual D.5.7.2 it appears that
>>>> va*, vaa* versions invalidate intermediate caching of
>>>> translation structures.
>>>>
>>>> With stage2 enabled that may result in 20+ memory lookups
>>>> for a 4 level page table walk. That's assuming that intermediate
>>>> caching structures cache mappings from stage1 table entry to
>>>> host page.
>>>
>>> Yeah, Catalin and I discussed improving the kernel support for this,
>>> but it requires some changes to the generic mmu_gather code so that we
>>> can distinguish the leaf cases. I'd also like to see that done in a way
>>> that takes into account different granule sizes (we currently iterate
>>> over huge pages in 4k chunks). Last time I touched that, I entered a
>>> world of pain and don't plan to return there immediately :)
>>>
>>> Catalin -- feeling brave?
>>>
>>> FWIW: the new IOMMU page-table stuff I just got merged *does* make use
>>> of leaf-invalidation for the SMMU.
>>
>> Now, talking about feeling brave: who will be silly enough to port KVM
>> to the IOMMU page table code? It should just work(tm), right?
> 
> I suspect you'll need to do some surgery to the interfaces, which currently
> map directly onto the IOMMU API and therefore make nice assumptions about
> what we get asked to map/unmap. You also probably want a wider range of
> permissions than we use on the SMMU. Finally, the runtime nature of the
> code (we make no assumptions about address sizes, page sizes etc) probably
> incurs a performance hit that you may or may not care about.

That's exactly what I want to evaluate. It would also help us to
decouple our page-table code from the kernel macros, which bite us time
and time again...

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...