On 2020/7/9 2:24, Catalin Marinas wrote:

On Wed, Jul 08, 2020 at 08:40:31PM +0800, Zhenyu Ye wrote:

quoted

Add __TLBI_VADDR_RANGE macro and rewrite __flush_tlb_range().

In this patch, we only use the TLBI RANGE feature if the stride == PAGE_SIZE,
because when stride > PAGE_SIZE, usually only a small number of pages need
to be flushed and classic tlbi intructions are more effective.

Why are they more effective? I guess a range op would work on this as
well, say unmapping a large THP range. If we ignore this stride ==
PAGE_SIZE, it could make the code easier to read.

OK, I will remove the stride == PAGE_SIZE here.

quoted

We can also use 'end - start < threshold number' to decide which way
to go, however, different hardware may have different thresholds, so
I'm not sure if this is feasible.

Signed-off-by: Zhenyu Ye <redacted>
---
 arch/arm64/include/asm/tlbflush.h | 104 ++++++++++++++++++++++++++----
 1 file changed, 90 insertions(+), 14 deletions(-)

Could you please rebase these patches on top of the arm64 for-next/tlbi
branch:

git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/tlbi

OK, I will send a formal version patch of this series soon.

quoted

 
-	if ((end - start) >= (MAX_TLBI_OPS * stride)) {
+	if ((!cpus_have_const_cap(ARM64_HAS_TLBI_RANGE) &&
+	    (end - start) >= (MAX_TLBI_OPS * stride)) ||
+	    range_pages >= MAX_TLBI_RANGE_PAGES) {
 		flush_tlb_mm(vma->vm_mm);
 		return;
 	}

Is there any value in this range_pages check here? What's the value of
MAX_TLBI_RANGE_PAGES? If we have TLBI range ops, we make a decision here
but without including the stride. Further down we use the stride to skip
the TLBI range ops.

MAX_TLBI_RANGE_PAGES is defined as __TLBI_RANGE_PAGES(31, 3), which is
decided by ARMv8.4 spec. The address range is determined by below formula:

	[BADDR, BADDR + (NUM + 1) * 2^(5*SCALE + 1) * PAGESIZE)

Which has nothing to do with the stride.  After removing the stride ==
PAGE_SIZE below, there will be more clear.

quoted

 }

I think the algorithm is correct, though I need to work it out on a
piece of paper.

The code could benefit from some comments (above the loop) on how the
range is built and the right scale found.

OK.

Thanks,
Zhenyu


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