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Subject[PATCH 3/3] arm64: compat: Fix __arch_get_hw_counter() implementation
Date
Provide the following fixes for the __arch_get_hw_counter()
implementation on arm64:
- Fallback on syscall when an unstable counter is detected.
- Introduce isb()s before and after the counter read to avoid
speculation of the counter value and of the seq lock
respectively.
The second isb() is a temporary solution that will be revisited
in 5.3-rc1.

These fixes restore the semantics that __arch_counter_get_cntvct()
had on arm64.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
.../include/asm/vdso/compat_gettimeofday.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/arch/arm64/include/asm/vdso/compat_gettimeofday.h b/arch/arm64/include/asm/vdso/compat_gettimeofday.h
index 93dbd935b66d..f4812777f5c5 100644
--- a/arch/arm64/include/asm/vdso/compat_gettimeofday.h
+++ b/arch/arm64/include/asm/vdso/compat_gettimeofday.h
@@ -12,6 +12,8 @@

#include <asm/vdso/compat_barrier.h>

+#define __VDSO_USE_SYSCALL ULLONG_MAX
+
#define VDSO_HAS_CLOCK_GETRES 1

static __always_inline
@@ -74,8 +76,24 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode)
{
u64 res;

+ /*
+ * clock_mode == 0 implies that vDSO are enabled otherwise
+ * fallback on syscall.
+ */
+ if (clock_mode)
+ return __VDSO_USE_SYSCALL;
+
+ /*
+ * This isb() is required to prevent that the counter value
+ * is speculated.
+ */
isb();
asm volatile("mrrc p15, 1, %Q0, %R0, c14" : "=r" (res));
+ /*
+ * This isb() is required to prevent that the seq lock is
+ * speculated.
+ */
+ isb();

return res;
}
--
2.22.0
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