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

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Date:   Fri, 16 Jul 2021 11:02:48 +0100
From:   Steven Price <steven.price@....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, 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 16/07/2021 08:20, Anshuman Khandual wrote:
> 
> 
> 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)'
> 

Ah, I'd only tested this for 4k. 16k would require a different set of masks.

So the bits we need to cover are those from just below PAGE_SHIFT to the
top of PTE_ADDR_HIGH (bit 10). So we can compute the mask for both 4k
and 16k with GENMASK(PAGE_SHIFT-1, 10):

	and	\pte, \pte, #PTE_ADDR_MASK | GENMASK(PAGE_SHIFT - 1, 10)
	and	\pte, \pte, #~GENMASK(PAGE_SHIFT - 1, 10)

This compiles (for both 4k and 16k) and the assembly looks correct, but
I've not done any other testing.

Steve