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Subject[PATCH v6 2/7] arm64: trap to EL1 accesses to AMU counters from EL0
Date
The activity monitors extension is an optional extension introduced
by the ARMv8.4 CPU architecture. In order to access the activity
monitors counters safely, if desired, the kernel should detect the
presence of the extension through the feature register, and mediate
the access.

Therefore, disable direct accesses to activity monitors counters
from EL0 (userspace) and trap them to EL1 (kernel).

To be noted that the ARM64_AMU_EXTN kernel config does not have an
effect on this code. Given that the amuserenr_el0 resets to an
UNKNOWN value, setting the trap of EL0 accesses to EL1 is always
attempted for safety and security considerations. Therefore firmware
should still ensure accesses to AMU registers are not trapped in
EL2/EL3 as this code cannot be bypassed if the CPU implements the
Activity Monitors Unit.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Steve Capper <steve.capper@arm.com>
---
arch/arm64/include/asm/assembler.h | 10 ++++++++++
arch/arm64/mm/proc.S | 3 +++
2 files changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index aca337d79d12..c5487806273f 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -430,6 +430,16 @@ USER(\label, ic ivau, \tmp2) // invalidate I line PoU
9000:
.endm

+/*
+ * reset_amuserenr_el0 - reset AMUSERENR_EL0 if AMUv1 present
+ */
+ .macro reset_amuserenr_el0, tmpreg
+ mrs \tmpreg, id_aa64pfr0_el1 // Check ID_AA64PFR0_EL1
+ ubfx \tmpreg, \tmpreg, #ID_AA64PFR0_AMU_SHIFT, #4
+ cbz \tmpreg, .Lskip_\@ // Skip if no AMU present
+ msr_s SYS_AMUSERENR_EL0, xzr // Disable AMU access from EL0
+.Lskip_\@:
+ .endm
/*
* copy_page - copy src to dest using temp registers t1-t8
*/
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index aafed6902411..7103027b4e64 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -131,6 +131,7 @@ alternative_endif
ubfx x11, x11, #1, #1
msr oslar_el1, x11
reset_pmuserenr_el0 x0 // Disable PMU access from EL0
+ reset_amuserenr_el0 x0 // Disable AMU access from EL0

alternative_if ARM64_HAS_RAS_EXTN
msr_s SYS_DISR_EL1, xzr
@@ -423,6 +424,8 @@ SYM_FUNC_START(__cpu_setup)
isb // Unmask debug exceptions now,
enable_dbg // since this is per-cpu
reset_pmuserenr_el0 x0 // Disable PMU access from EL0
+ reset_amuserenr_el0 x0 // Disable AMU access from EL0
+
/*
* Memory region attributes
*/
--
2.17.1
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