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

[Zhenyu Ye <yezhenyu2@huawei.com>]

Hi James,

On 2020/3/26 0:15, James Morse wrote:
> 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!
> 

Thanks for your review.  With THP, we should update the TTL value
after the page collapse and merge.  If not sure what it should be,
we can set it to 0 to avoid "no TLB maintenance occur" problem.
The Table D5-53 in DDI0487E.a says:
| when TTL[1:0] is 0b00:
|   This value is reserved, and hardware should treat this as if TTL[3:2] is 0b00
| when TTL[3:2] is 0b00:
|   Hardware must assume that the entry can be from any level.

> 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())
> 

I will try to add struct mmu_gather to TLBI interfaces, which has enough
info to track tlb's level.  See in next patch version!


Thanks,
Zhenyu

.