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

[Will Deacon]

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