linux-kernel - Re: [PATCH] arm64: tlbflush: avoid writing RES0 bits

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Date:   Mon, 26 Mar 2018 20:51:45 -0400
From:   Philip Elcan <pelcan@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org,
        Thomas Speier <tspeier@...eaurora.org>,
        Shanker Donthineni <shankerd@...eaurora.org>
Subject: Re: [PATCH] arm64: tlbflush: avoid writing RES0 bits

On 3/26/2018 6:02 AM, Mark Rutland wrote:
> On Wed, Mar 21, 2018 at 05:02:52PM -0400, Philip Elcan wrote:
>> Bits [47:44] of the TLBI register operand are RES0 for instructions that
>> require a VA, per the ARM ARM spec, so TLBI operations should avoid writing
>> non-zero values to these bits.
>>
>> Signed-off-by: Philip Elcan <pelcan@...eaurora.org>
>> ---
>>  arch/arm64/include/asm/tlbflush.h | 16 ++++++++++------
>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>> index 9e82dd7..dbd22a9 100644
>> --- a/arch/arm64/include/asm/tlbflush.h
>> +++ b/arch/arm64/include/asm/tlbflush.h
>> @@ -60,6 +60,9 @@
>>  		__tlbi(op, (arg) | USER_ASID_FLAG);				\
>>  } while (0)
>>  
>> +/* This macro masks out RES0 bits in the TLBI operand */
>> +#define __TLBI_VADDR(addr) (addr & ~GENMASK_ULL(47, 44))
> 
> If we're going to mask the address bits, it would be simpler to keep the
> valid bits than to clear the invalid bits. i.e.
> 
> #define __TLBI_VADDR(addr) (addr & GENMASK_ULL(43, 0))
> 
> Maybe we want a helper that does all of the addr / asid shifting and
> masking, so we do that in one place rather than spreading it across all
> helpers, e.g.
> 
> #define __tlbi_addr(addr, asid)				\
> ({							\
> 	unsigned long __ta = (addr) >> 12;		\
> 	__ta &= GENMASK_ULL(43, 0);			\
> 	__ta |= (asid) << 48;				\
> 	__ta;						\
> })
> 
> Thanks,
> Mark.
> 

That makes sense and addresses Robin's comment as well. I'll send out a
v2 with your suggestion.

Thanks,
Philip

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