linux-kernel - [PATCH] arm64: vdso: Use __arch_counter_get

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Message-Id: <20250407-arm-vdso-v1-1-7012de25b195@debian.org>
Date: Mon, 07 Apr 2025 04:33:25 -0700
From: Breno Leitao <leitao@...ian.org>
To: Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>, maz@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com, Breno Leitao <leitao@...ian.org>
Subject: [PATCH] arm64: vdso: Use __arch_counter_get_cntvct()

While reading how `cntvct_el0` was read in the kernel, I found that
__arch_get_hw_counter() is doing something very similar to what
__arch_counter_get_cntvct() is already doing.

Use the existing __arch_counter_get_cntvct() function instead of
duplicating similar inline assembly code in __arch_get_hw_counter().

Both functions were performing nearly identical operations to read the
cntvct_el0 register. The only difference was that
__arch_get_hw_counter() included a memory clobber in its inline
assembly, which appears unnecessary in this context.

This change simplifies the code by eliminating duplicate functionality
and improves maintainability by centralizing the counter access logic in
a single implementation.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
Changes from RFC:
- Move the #include files to be alphabetically ordered
- Link to RFC: https://lore.kernel.org/r/20250402-arm-vdso-v1-1-2e7a12d75107@debian.org
---
 arch/arm64/include/asm/vdso/gettimeofday.h | 22 ++--------------------
 1 file changed, 2 insertions(+), 20 deletions(-)

diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
index 92a2b59a9f3df..83c315ed5c6e7 100644
--- a/arch/arm64/include/asm/vdso/gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/gettimeofday.h
@@ -8,6 +8,7 @@
 #ifndef __ASSEMBLY__
 
 #include <asm/alternative.h>
+#include <asm/arch_timer.h>
 #include <asm/barrier.h>
 #include <asm/unistd.h>
 #include <asm/sysreg.h>
@@ -69,8 +70,6 @@ int clock_getres_fallback(clockid_t _clkid, struct __kernel_timespec *_ts)
 static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
 						 const struct vdso_time_data *vd)
 {
-	u64 res;
-
 	/*
 	 * Core checks for mode already, so this raced against a concurrent
 	 * update. Return something. Core will do another round and then
@@ -79,24 +78,7 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
 	if (clock_mode == VDSO_CLOCKMODE_NONE)
 		return 0;
 
-	/*
-	 * If FEAT_ECV is available, use the self-synchronizing counter.
-	 * Otherwise the isb is required to prevent that the counter value
-	 * is speculated.
-	*/
-	asm volatile(
-	ALTERNATIVE("isb\n"
-		    "mrs %0, cntvct_el0",
-		    "nop\n"
-		    __mrs_s("%0", SYS_CNTVCTSS_EL0),
-		    ARM64_HAS_ECV)
-	: "=r" (res)
-	:
-	: "memory");
-
-	arch_counter_enforce_ordering(res);
-
-	return res;
+	return __arch_counter_get_cntvct();
 }
 
 #endif /* !__ASSEMBLY__ */

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250402-arm-vdso-c4b0c9f35300

Best regards,
-- 
Breno Leitao <leitao@...ian.org>