Re: [PATCH V2] arm64/mm: Re-organise setting up FEAT_S1PIE registers PIRE0_EL1 and PIR_EL1

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/24/25 15:37, Ryan Roberts wrote:
> On 16/04/2025 04:56, Anshuman Khandual wrote:
>> mov_q cannot really move PIE_E[0|1] macros into a general purpose register
>> as expected if those macro constants contain some 128 bit layout elements,
>> that are required for D128 page tables. The primary issue is that for D128,
>> PIE_E[0|1] are defined in terms of 128-bit types with shifting and masking,
>> which the assembler can't accommodate.
>>
>> Instead pre-calculate these PIRE0_EL1/PIR_EL1 constants into asm-offsets.h
>> based PIE_E0_ASM/PIE_E1_ASM which can then be used in arch/arm64/mm/proc.S.
>>
>> While here also move PTE_MAYBE_NG/PTE_MAYBE_SHARED assembly overrides into
>> arch/arm64/kernel/asm-offsets.c to ensure PIRE0_EL1/PIR_EL1 are calculated
>> in assembly without arm64_use_ng_mappings and lpa2_is_enabled() symbols
>> being accessible. Also move the corresponding comment as well.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Ard Biesheuvel <ardb@kernel.org>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This patch applies on v6.15-rc2
>>
>> Changes in V2:
>>
>> - Added asm-offsets.c based PIE_E0_ASM and PIE_E1_ASM symbols as per Ard
>> - Moved PTE_MAYBE_NG and PTE_MAYBE_SHARED overrides inside asm-offsets.c
>>   along with the corresponding comment as per Ard
>>
>> Changes in V1:
>>
>> https://lore.kernel.org/linux-arm-kernel/20250410074024.1545768-1-anshuman.khandual@arm.com/
>>
>>  arch/arm64/kernel/asm-offsets.c | 16 ++++++++++++++++
>>  arch/arm64/mm/proc.S            | 19 ++-----------------
>>  2 files changed, 18 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
>> index eb1a840e4110..5b99a78f6882 100644
>> --- a/arch/arm64/kernel/asm-offsets.c
>> +++ b/arch/arm64/kernel/asm-offsets.c
>> @@ -182,5 +182,21 @@ int main(void)
>>  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>>    DEFINE(FTRACE_OPS_DIRECT_CALL,	offsetof(struct ftrace_ops, direct_call));
>>  #endif
>> +	/*
>> +	 * The PROT_* macros describing the various memory types may resolve to
>> +	 * C expressions if they include the PTE_MAYBE_* macros, and so they
>> +	 * can only be used from C code. The PIE_E* constants below are also
>> +	 * defined in terms of those macros, but will mask out those
>> +	 * PTE_MAYBE_* constants, whether they are set or not. So #define them
>> +	 * as 0x0 here so we can evaluate the PIE_E* constants in asm context.
>> +	 */
>> +#undef PTE_MAYBE_NG
>> +#define PTE_MAYBE_NG		0
>> +
>> +#undef PTE_MAYBE_SHARED
>> +#define PTE_MAYBE_SHARED	0
> 
> My toolchain at least is smart enough to figure out that the bits of interest in
> PIE_E1 are all contstant so it works without this hack.
> 
> I'd prefer to drop this hack on that basis. Or if there are issues with other
> toolchains that mean we need to keep it, I think the wording of the comment

So could we keep these for some more time and later drop them off just to be
on the safer side ?

> should be changed since we are now in C code so the "can only be used from C
> code" bit doesn't really make sense.

Dropped that. Does this look better ?

	/*
	 * The PROT_* macros describing the various memory types may resolve
	 * to C expressions if they include the PTE_MAYBE_* macros. The PIE_E*
	 * constants below are also defined in terms of those macros, but will
	 * mask out those PTE_MAYBE_* constants, whether they are set or not.
	 * So #define them as 0x0 here so we can evaluate the PIE_E* constants
	 * in asm context.
 	 */

> 
> Thanks,
> Ryan
> 
>> +
>> +  DEFINE(PIE_E0_ASM, PIE_E0);
>> +  DEFINE(PIE_E1_ASM, PIE_E1);
>>    return 0;
>>  }
>> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
>> index fb30c8804f87..80d470aa469d 100644
>> --- a/arch/arm64/mm/proc.S
>> +++ b/arch/arm64/mm/proc.S
>> @@ -512,26 +512,11 @@ alternative_else_nop_endif
>>  	ubfx	x1, x1, #ID_AA64MMFR3_EL1_S1PIE_SHIFT, #4
>>  	cbz	x1, .Lskip_indirection
>>  
>> -	/*
>> -	 * The PROT_* macros describing the various memory types may resolve to
>> -	 * C expressions if they include the PTE_MAYBE_* macros, and so they
>> -	 * can only be used from C code. The PIE_E* constants below are also
>> -	 * defined in terms of those macros, but will mask out those
>> -	 * PTE_MAYBE_* constants, whether they are set or not. So #define them
>> -	 * as 0x0 here so we can evaluate the PIE_E* constants in asm context.
>> -	 */
>> -
>> -#define PTE_MAYBE_NG		0
>> -#define PTE_MAYBE_SHARED	0
>> -
>> -	mov_q	x0, PIE_E0
>> +	mov_q	x0, PIE_E0_ASM
>>  	msr	REG_PIRE0_EL1, x0
>> -	mov_q	x0, PIE_E1
>> +	mov_q	x0, PIE_E1_ASM
>>  	msr	REG_PIR_EL1, x0
>>  
>> -#undef PTE_MAYBE_NG
>> -#undef PTE_MAYBE_SHARED
>> -
>>  	orr	tcr2, tcr2, TCR2_EL1_PIE
>>  	msr	REG_TCR2_EL1, x0
>>  
>