linux-kernel - Re: [RFC 07/10] arm64/mm: Detect and enable FEAT

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Date:   Fri, 16 Jul 2021 09:08:57 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, mark.rutland@....com, will@...nel.org,
        catalin.marinas@....com, maz@...nel.org, james.morse@....com,
        steven.price@....com
Subject: Re: [RFC 07/10] arm64/mm: Detect and enable FEAT_LPA2

On 16/07/2021 08:06, Anshuman Khandual wrote:
> 
> On 7/14/21 1:51 PM, Suzuki K Poulose wrote:
>> On 14/07/2021 03:21, Anshuman Khandual wrote:
>>> Detect FEAT_LPA2 implementation early enough during boot when requested via
>>> CONFIG_ARM64_PA_BITS_52_LPA2 and remember in a variable arm64_lpa2_enabled.
>>> This variable could then be used to turn on TCR_EL1.TCR_DS effecting the 52
>>> bits PA range or fall back to default 48 bits PA range if FEAT_LPA2 feature
>>> was requested but found not to be implemented.
>>>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>>    arch/arm64/include/asm/memory.h |  1 +
>>>    arch/arm64/kernel/head.S        | 15 +++++++++++++++
>>>    arch/arm64/mm/mmu.c             |  3 +++
>>>    arch/arm64/mm/proc.S            |  9 +++++++++
>>>    4 files changed, 28 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>>> index 824a365..d0ca002 100644
>>> --- a/arch/arm64/include/asm/memory.h
>>> +++ b/arch/arm64/include/asm/memory.h
>>> @@ -178,6 +178,7 @@
>>>    #include <asm/bug.h>
>>>      extern u64            vabits_actual;
>>> +extern u64            arm64_lpa2_enabled;
>>>      extern s64            memstart_addr;
>>>    /* PHYS_OFFSET - the physical address of the start of memory. */
>>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>>> index 6444147..9cf79ea 100644
>>> --- a/arch/arm64/kernel/head.S
>>> +++ b/arch/arm64/kernel/head.S
>>> @@ -94,6 +94,21 @@ SYM_CODE_START(primary_entry)
>>>        adrp    x23, __PHYS_OFFSET
>>>        and    x23, x23, MIN_KIMG_ALIGN - 1    // KASLR offset, defaults to 0
>>>        bl    set_cpu_boot_mode_flag
>>> +
>>> +#ifdef CONFIG_ARM64_PA_BITS_52_LPA2
>>> +    mrs     x10, ID_AA64MMFR0_EL1
>>> +    ubfx    x10, x10, #ID_AA64MMFR0_TGRAN_SHIFT, 4
>>> +    cmp     x10, #ID_AA64MMFR0_TGRAN_LPA2
>>> +    b.ne    1f
>>
>> For the sake of forward compatibility, this should be "b.lt"
> Right, I guess we could assume that the feature will be present from the
> current ID_AA64MMFR0_TGRAN_LPA2 values onward in the future. But should
> not this also be capped at ID_AA64MMFR0_TGRAN_SUPPORTED_MAX as the upper
> limit is different for 4K and 16K page sizes.

Absolutely.

Cheers
Suzuki

>