linux-kernel - Re: [RFC 06/10] arm64/mm: Add FEAT

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Date:   Fri, 16 Jul 2021 12:50:49 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Steven Price <steven.price@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, suzuki.poulose@....com,
        mark.rutland@....com, will@...nel.org, catalin.marinas@....com,
        maz@...nel.org, james.morse@....com
Subject: Re: [RFC 06/10] arm64/mm: Add FEAT_LPA2 specific encoding



On 7/14/21 9:08 PM, Steven Price wrote:
> On 14/07/2021 03:21, Anshuman Khandual wrote:
>> FEAT_LPA2 requires different PTE representation formats for both 4K and 16K
>> page size config. This adds FEAT_LPA2 specific new PTE encodings as per ARM
>> ARM (0487G.A) which updates [pte|phys]_to_[phys|pte](). The updated helpers
>> would be used when FEAT_LPA2 gets enabled via CONFIG_ARM64_PA_BITS_52 on 4K
>> and 16K page size. Although TTBR encoding and phys_to_ttbr() helper remains
>> the same as FEAT_LPA for FEAT_LPA2 as well. It updates 'phys_to_pte' helper
>> to accept a temporary variable and changes impacted call sites.
>>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  arch/arm64/include/asm/assembler.h     | 23 +++++++++++++++++++----
>>  arch/arm64/include/asm/pgtable-hwdef.h |  4 ++++
>>  arch/arm64/include/asm/pgtable.h       |  4 ++++
>>  arch/arm64/kernel/head.S               | 25 +++++++++++++------------
>>  4 files changed, 40 insertions(+), 16 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
>> index fedc202..0492543 100644
>> --- a/arch/arm64/include/asm/assembler.h
>> +++ b/arch/arm64/include/asm/assembler.h
>> @@ -606,7 +606,7 @@ alternative_endif
>>  #endif
>>  	.endm
>>  
>> -	.macro	phys_to_pte, pte, phys
>> +	.macro	phys_to_pte, pte, phys, tmp
>>  #ifdef CONFIG_ARM64_PA_BITS_52_LPA
>>  	/*
>>  	 * We assume \phys is 64K aligned and this is guaranteed by only
>> @@ -614,6 +614,17 @@ alternative_endif
>>  	 */
>>  	orr	\pte, \phys, \phys, lsr #36
>>  	and	\pte, \pte, #PTE_ADDR_MASK
>> +#elif defined(CONFIG_ARM64_PA_BITS_52_LPA2)
>> +	orr	\pte, \phys, \phys, lsr #42
>> +
>> +	/*
>> +	 * The 'tmp' is being used here to just prepare
>> +	 * and hold PTE_ADDR_MASK which cannot be passed
>> +	 * to the subsequent 'and' instruction.
>> +	 */
>> +	mov	\tmp, #PTE_ADDR_LOW
>> +	orr	\tmp, \tmp, #PTE_ADDR_HIGH
>> +	and	\pte, \pte, \tmp
> Rather than adding an extra temporary register (and the fallout of
> various other macros needing an extra register), this can be done with
> two AND instructions:

I would really like to get rid of the 'tmp' variable here as
well but did not figure out any method of accomplishing it.

> 
> 	/* PTE_ADDR_MASK cannot be encoded as an immediate, so
>          * mask off all but two bits, followed by masking the
>          * extra two bits
>          */
> 	and	\pte, \pte, #PTE_ADDR_MASK | (3 << 10)
> 	and	\pte, \pte, #~(3 << 10)

Did this change as suggested

--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -626,9 +626,8 @@ alternative_endif
         * and hold PTE_ADDR_MASK which cannot be passed
         * to the subsequent 'and' instruction.
         */
-       mov     \tmp, #PTE_ADDR_LOW
-       orr     \tmp, \tmp, #PTE_ADDR_HIGH
-       and     \pte, \pte, \tmp
+       and     \pte, \pte, #PTE_ADDR_MASK | (0x3 << 10)
+       and     \pte, \pte, #~(0x3 << 10)
 
 .Lskip_lpa2\@:
        mov     \pte, \phys


but still fails to build (tested on 16K)

arch/arm64/kernel/head.S: Assembler messages:
arch/arm64/kernel/head.S:377: Error: immediate out of range at operand 3 -- `and x6,x6,#((((1<<(50-14))-1)<<14)|(0x3<<8))|(0x3<<10)'
arch/arm64/kernel/head.S:390: Error: immediate out of range at operand 3 -- `and x12,x12,#((((1<<(50-14))-1)<<14)|(0x3<<8))|(0x3<<10)'
arch/arm64/kernel/head.S:390: Error: immediate out of range at operand 3 -- `and x12,x12,#((((1<<(50-14))-1)<<14)|(0x3<<8))|(0x3<<10)'
arch/arm64/kernel/head.S:404: Error: immediate out of range at operand 3 -- `and x12,x12,#((((1<<(50-14))-1)<<14)|(0x3<<8))|(0x3<<10)'
arch/arm64/kernel/head.S:404: Error: immediate out of range at operand 3 -- `and x12,x12,#((((1<<(50-14))-1)<<14)|(0x3<<8))|(0x3<<10)'