[RFC] arm64: Ensure proper addressing for ldnp/stnp

[bdegraaf@codeaurora.org (Brent DeGraaf)]

According to section 6.3.8 of the ARM Programmer's Guide, non-temporal
loads and stores do not verify that address dependency is met between a
load of an address to a register and a subsequent non-temporal load or
store using that address on the executing PE. Therefore, context switch
code and subroutine calls that use non-temporally accessed addresses as
parameters that might depend on a load of an address into an argument
register must ensure that ordering requirements are met by introducing
a barrier prior to the successive non-temporal access.  Add appropriate
barriers whereever this specific situation comes into play.

Signed-off-by: Brent DeGraaf <bdegraaf@codeaurora.org>
---
 arch/arm64/kernel/entry.S  | 1 +
 arch/arm64/lib/copy_page.S | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 441420c..982c4d3 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -679,6 +679,7 @@ ENTRY(cpu_switch_to)
 	ldp	x27, x28, [x8], #16
 	ldp	x29, x9, [x8], #16
 	ldr	lr, [x8]
+	dmb	nshld	// Existence of instructions with loose load-use dependencies (e.g. ldnp/stnp) make this barrier necessary
 	mov	sp, x9
 	and	x9, x9, #~(THREAD_SIZE - 1)
 	msr	sp_el0, x9
diff --git a/arch/arm64/lib/copy_page.S b/arch/arm64/lib/copy_page.S
index 4c1e700..21c6892 100644
--- a/arch/arm64/lib/copy_page.S
+++ b/arch/arm64/lib/copy_page.S
@@ -47,6 +47,8 @@ alternative_endif
 	ldp	x14, x15, [x1, #96]
 	ldp	x16, x17, [x1, #112]
 
+	dmb	nshld // In case x0 (for stnp) is dependent on a load
+
 	mov	x18, #(PAGE_SIZE - 128)
 	add	x1, x1, #128
 1:
-- 
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