Re: [RFC PATCH v2] arm64: cpufeatures: add support for tlbi range instructions

[Zhenyu Ye]

On 2019/11/11 21:27, Will Deacon wrote:
> On Mon, Nov 11, 2019 at 09:23:55PM +0800, Zhenyu Ye wrote:
>> ARMv8.4-TLBI provides TLBI invalidation instruction that apply to a
>> range of input addresses. This patch adds support for this feature.
>> This is the second version of the patch.
>>
>> I traced the __flush_tlb_range() for a minute and get some statistical
>> data as below:
>>
>> 	PAGENUM		COUNT
>> 	1		34944
>> 	2		5683
>> 	3		1343
>> 	4		7857
>> 	5		838
>> 	9		339
>> 	16		933
>> 	19		427
>> 	20		5821
>> 	23		279
>> 	41		338
>> 	141		279
>> 	512		428
>> 	1668		120
>> 	2038		100
>>
>> Those data are based on kernel-5.4.0, where PAGENUM = end - start, COUNT
>> shows number of calls to the __flush_tlb_range() in a minute. There only
>> shows the data which COUNT >= 100. The kernel is started normally, and
>> transparent hugepage is opened. As we can see, though most user TLBI
>> ranges were 1 pages long, the num of long-range can not be ignored.
>>
>> The new feature of TLB range can improve lots of performance compared to
>> the current implementation. As an example, flush 512 ranges needs only 1
>> instruction as opposed to 512 instructions using current implementation.
>>
>> And for a new hardware feature, support is better than not.
>>
>> Signed-off-by: Zhenyu Ye <yezhenyu2@xxxxxxxxxx>
>> ---
>> ChangeLog v1 -> v2:
>> - Change the main implementation of this feature.
>> - Add some comments.
> 
> How does this address my concerns here:
> 
> https://lore.kernel.org/linux-arm-kernel/20191031131649.GB27196@willie-the-truck/
> 
> ?
> 
> Will
> 
> .
> 

I think your concern is more about the hardware level, and we can do nothing about
this at all. The interconnect/DVM implementation is not exposed to software layer
(and no need), and may should be constrained at hardware level.

Zhenyu