[RFC PATCH] arm64: Elide dsb in kernel TLB invalidations

[Dev Jain <dev.jain@arm.com>]

dsb(ishst) is used to ensure that prior pagetable updates are completed.
But, set_pmd/set_pud etc already issue a dsb-isb sequence for the exact
same purpose. Therefore, we can elide the dsb in kernel tlb invalidation.

There were no issues observed while running mm selftests, including
test_vmalloc.sh selftest to stress the vmalloc subsystem.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 arch/arm64/include/asm/tlbflush.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
index eba1a98657f1..9b4adf1ee45e 100644
--- a/arch/arm64/include/asm/tlbflush.h
+++ b/arch/arm64/include/asm/tlbflush.h
@@ -508,7 +508,7 @@ static inline void flush_tlb_kernel_range(unsigned long start, unsigned long end
 		return;
 	}
 
-	dsb(ishst);
+	/* dsb(ishst) not needed as callers (set_pxd) have that */
 	__flush_tlb_range_op(vaale1is, start, pages, stride, 0,
 			     TLBI_TTL_UNKNOWN, false, lpa2_is_enabled());
 	dsb(ish);
@@ -523,7 +523,7 @@ static inline void __flush_tlb_kernel_pgtable(unsigned long kaddr)
 {
 	unsigned long addr = __TLBI_VADDR(kaddr, 0);
 
-	dsb(ishst);
+	/* dsb(ishst) not needed as callers (set_pxd) have that */
 	__tlbi(vaae1is, addr);
 	dsb(ish);
 	isb();
-- 
2.30.2