Re: [RFC PATCH v4 0/6] arm64: tlb: add support for TTL feature

[James Morse]

Hi Zhenyu,

On 3/24/20 1:45 PM, Zhenyu Ye wrote:
> In order to reduce the cost of TLB invalidation, the ARMv8.4 TTL
> feature allows TLBs to be issued with a level allowing for quicker
> invalidation.  This series provide support for this feature. 
> 
> Patch 1 and Patch 2 was provided by Marc on his NV series[1] patches,
> which detect the TTL feature and add __tlbi_level interface.

How does this interact with THP?
(I don't see anything on that in the series.)

With THP, there is no one answer to the size of mapping in a VMA.
This is a problem because the arm-arm has in "Translation table level
hints" in D5.10.2 of DDI0487E.a:
| If an incorrect value for the entry being invalidated by the
| instruction is specified in the TTL field, then no entries are
| required by the architecture to be invalidated from the TLB.

If we get it wrong, not TLB maintenance occurs!

Unless THP leaves its fingerprints on the vma, I think you can only do
this for VMA types that THP can't mess with. (see
transparent_hugepage_enabled())


Thanks,

James