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Message-ID: <1b15b4f0-5490-4dac-8344-e716dd189751@arm.com>
Date: Tue, 15 Jul 2025 11:08:48 +0530
From: Dev Jain <dev.jain@....com>
To: Ryan Roberts <ryan.roberts@....com>, Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
Catalin Marinas <catalin.marinas@....com>,
Mark Rutland <mark.rutland@....com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH 09/10] arm64: mm: Simplify
__flush_tlb_range_limit_excess()
On 14/07/25 3:00 pm, Ryan Roberts wrote:
> On 11/07/2025 17:17, Will Deacon wrote:
>> __flush_tlb_range_limit_excess() is unnecessarily complicated:
>>
>> - It takes a 'start', 'end' and 'pages' argument, whereas it only
>> needs 'pages' (which the caller has computed from the other two
>> arguments!).
>>
>> - It erroneously compares 'pages' with MAX_TLBI_RANGE_PAGES when
>> the system doesn't support range-based invalidation but the range to
>> be invalidated would result in fewer than MAX_DVM_OPS invalidations.
>>
>> Simplify the function so that it no longer takes the 'start' and 'end'
>> arguments and only considers the MAX_TLBI_RANGE_PAGES threshold on
>> systems that implement range-based invalidation.
>>
>> Signed-off-by: Will Deacon <will@...nel.org>
> Does this warrant a Fixes: tag?
>
>> ---
>> arch/arm64/include/asm/tlbflush.h | 20 ++++++--------------
>> 1 file changed, 6 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>> index 8618a85d5cd3..2541863721af 100644
>> --- a/arch/arm64/include/asm/tlbflush.h
>> +++ b/arch/arm64/include/asm/tlbflush.h
>> @@ -470,21 +470,13 @@ do { \
>> #define __flush_s2_tlb_range_op(op, start, pages, stride, tlb_level) \
>> __flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, kvm_lpa2_is_enabled());
>>
>> -static inline bool __flush_tlb_range_limit_excess(unsigned long start,
>> - unsigned long end, unsigned long pages, unsigned long stride)
>> +static inline bool __flush_tlb_range_limit_excess(unsigned long pages,
>> + unsigned long stride)
>> {
>> - /*
>> - * When the system does not support TLB range based flush
>> - * operation, (MAX_DVM_OPS - 1) pages can be handled. But
>> - * with TLB range based operation, MAX_TLBI_RANGE_PAGES
>> - * pages can be handled.
>> - */
>> - if ((!system_supports_tlb_range() &&
>> - (end - start) >= (MAX_DVM_OPS * stride)) ||
>> - pages > MAX_TLBI_RANGE_PAGES)
>> + if (system_supports_tlb_range() && pages > MAX_TLBI_RANGE_PAGES)
>> return true;
>>
>> - return false;
>> + return pages >= (MAX_DVM_OPS * stride) >> PAGE_SHIFT;
>> }
> I'm still not sure I totally get this... Aren't these really 2 separate
> concepts? MAX_TLBI_RANGE_PAGES is the max amount of VA that can be handled by a
> single tlbi-by-range (and due to implementation, the largest range that can be
> handled by the loop in __flush_tlb_range_op()). Whereas MAX_DVM_OPS is the max
> number of tlbi instrcutions you want to issue with the PTL held? Perhaps it is
> better to split these out; For the range case, calculate the number of ops you
> actually need and compare with MAX_DVM_OPS?
If system_supports_tlb_range() returns true and pages <= MAX_TLBI_RANGE_PAGES,
then it is guaranteed that the number of tlbi range operations issued is bounded
by 4 -> we start from scale = 3, and by patch 6 of this series (and the loop
itself btw) it is guaranteed that the scale will be decremented, so the worst
case is that all scales are used, so at most 4 operations will be issued, so
we are safe.
So if my reasoning is correct, I think what we need is something like:
diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
index aa9efee17277..53591caf3793 100644
--- a/arch/arm64/include/asm/tlbflush.h
+++ b/arch/arm64/include/asm/tlbflush.h
@@ -427,21 +427,19 @@ do { \
#define __flush_s2_tlb_range_op(op, start, pages, stride, tlb_level) \
__flush_tlb_range_op(op, start, pages, stride, 0, tlb_level, false, kvm_lpa2_is_enabled());
-static inline bool __flush_tlb_range_limit_excess(unsigned long start,
- unsigned long end, unsigned long pages, unsigned long stride)
+static inline bool __flush_tlb_range_limit_excess(unsigned long pages,
+ unsigned long stride)
{
/*
- * When the system does not support TLB range based flush
- * operation, (MAX_DVM_OPS - 1) pages can be handled. But
- * with TLB range based operation, MAX_TLBI_RANGE_PAGES
- * pages can be handled.
+ * If a TLBI range op has pages under MAX_TLBI_RANGE_PAGES, it
+ * is guaranteed that the loop in __flush_tlb_range_op shall
+ * terminate in at most 4 iterations, so we do not need to
+ * check with MAX_DVM_OPS in this case.
*/
- if ((!system_supports_tlb_range() &&
- (end - start) >= (MAX_DVM_OPS * stride)) ||
- pages > MAX_TLBI_RANGE_PAGES)
- return true;
+ if (system_supports_tlb_range())
+ return pages > MAX_TLBI_RANGE_PAGES;
- return false;
+ return pages >= (MAX_DVM_OPS * stride) >> PAGE_SHIFT;
}
static inline void __flush_tlb_range_nosync(struct mm_struct *mm,
where the comment can be worded better.
>
>
>>
>> static inline void __flush_tlb_range_nosync(struct mm_struct *mm,
>> @@ -498,7 +490,7 @@ static inline void __flush_tlb_range_nosync(struct mm_struct *mm,
>> end = round_up(end, stride);
>> pages = (end - start) >> PAGE_SHIFT;
>>
>> - if (__flush_tlb_range_limit_excess(start, end, pages, stride)) {
>> + if (__flush_tlb_range_limit_excess(pages, stride)) {
>> flush_tlb_mm(mm);
>> return;
>> }
>> @@ -547,7 +539,7 @@ static inline void flush_tlb_kernel_range(unsigned long start, unsigned long end
>> end = round_up(end, stride);
>> pages = (end - start) >> PAGE_SHIFT;
>>
>> - if (__flush_tlb_range_limit_excess(start, end, pages, stride)) {
>> + if (__flush_tlb_range_limit_excess(pages, stride)) {
>> flush_tlb_all();
>> return;
>> }
>
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