[PATCH] arm64: guard AMU register access with ARM64_HAS_AMU_EXTN

[Marek Vasut]

On 10/22/25 4:20 PM, Marc Zyngier wrote:
> On Wed, 22 Oct 2025 14:35:28 +0100,
> Marek Vasut <marek.vasut+renesas at mailbox.org> wrote:
>>
>> The AMU configuration register access may fault and prevent successful
>> kernel boot. This can occur for example in case the firmware does not
>> allow AMU counter access from EL1. Guard the AMU configuration register
>> access with ARM64_HAS_AMU_EXTN to prevent this fault if ARM64_HAS_AMU_EXTN
>> Kconfig option is explicitly disabled. Other interaction with the AMU is
>> already guarded by similar ifdeffery.
>>
>> Fixes: 87a1f063464a ("arm64: trap to EL1 accesses to AMU counters from EL0")
>> Signed-off-by: Marek Vasut <marek.vasut+renesas at mailbox.org>
>> ---
>> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
>> Cc: Catalin Marinas <catalin.marinas at arm.com>
>> Cc: Geert Uytterhoeven <geert+renesas at glider.be>
>> Cc: Marc Zyngier <maz at kernel.org>
>> Cc: Ryan Roberts <ryan.roberts at arm.com>
>> Cc: Will Deacon <will at kernel.org>
>> Cc: Yicong Yang <yangyicong at hisilicon.com>
>> Cc: linux-arm-kernel at lists.infradead.org
>> Cc: linux-renesas-soc at vger.kernel.org
>> ---
>>   arch/arm64/mm/proc.S | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
>> index 86818511962b6..123538ffeda6b 100644
>> --- a/arch/arm64/mm/proc.S
>> +++ b/arch/arm64/mm/proc.S
>> @@ -145,7 +145,9 @@ SYM_FUNC_START(cpu_do_resume)
>>   	ubfx	x11, x11, #1, #1
>>   	msr	oslar_el1, x11
>>   	reset_pmuserenr_el0 x0			// Disable PMU access from EL0
>> +alternative_if ARM64_HAS_AMU_EXTN
>>   	reset_amuserenr_el0 x0			// Disable AMU access from EL0
>> +alternative_else_nop_endif
> 
> Why?
> 
> We ensure that the AMU is available in the macro itself by checking
> for ID_AA64PFR0_EL1.AMU. If the AMu isn't present on this CPU, we skip
> the offending sysreg access. This is similar to what we do for the
> PMU.
> 
> Does your HW advertise an AMU, but doesn't actually have one?
The hardware does have AMU, but it is currently blocked in old TFA 
version and access to this AMU register here causes a fault. That's why 
I have to disable ARM64_HAS_AMU_EXTN until the TFA is updated and the 
AMU access is made available on this hardware. But even if I do disable 
ARM64_HAS_AMU_EXTN=n , I get a fault.

This patch is mainly meant to prevent a surprise in case someone does 
set ARM64_HAS_AMU_EXTN=n , and the system still faults on AMU register 
access.