linux-kernel - Re: [PATCH 06/10] ARC: [plat-eznps] Fix TLB Errata

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Date:   Thu, 1 Jun 2017 14:16:41 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "noamca@...lanox.com" <noamca@...lanox.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH 06/10] ARC: [plat-eznps] Fix TLB Errata

On 05/25/2017 04:00 AM, Alexey Brodkin wrote:
> Hi Noam,
> 
> On Thu, 2017-05-25 at 05:34 +0300, Noam Camus wrote:
>> From: Noam Camus <noamca@...lanox.com>
>>
>> Due to a HW bug in NPS400 we get from time to time false TLB miss.
>> Workaround this by validating each miss.
>>
>> Signed-off-by: Noam Camus <noamca@...lanox.com>
>> ---
>>   arch/arc/mm/tlbex.S |   10 ++++++++++
>>   1 files changed, 10 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/arc/mm/tlbex.S b/arch/arc/mm/tlbex.S
>> index b30e4e3..1d48723 100644
>> --- a/arch/arc/mm/tlbex.S
>> +++ b/arch/arc/mm/tlbex.S
>> @@ -274,6 +274,13 @@ ex_saved_reg1:
>>   .macro COMMIT_ENTRY_TO_MMU
>>   #if (CONFIG_ARC_MMU_VER < 4)
>>   
>> +#ifdef CONFIG_EZNPS_MTM_EXT
>> +	/* verify if entry for this vaddr+ASID already exists */
>> +	sr    TLBProbe, [ARC_REG_TLBCOMMAND]
>> +	lr    r0, [ARC_REG_TLBINDEX]
>> +	bbit0 r0, 31, 88f
>> +#endif
> 
> That's funny. I think we used to have something like that in the past.

Not here as this is fast path TLB refill handler and landign here implies entry 
was *not* present, unless there's a hardware bug, hence this patch.

Perhaps you are remembering the slow path TLB update code (tlb.c) which has always 
had this - as mm code can call update_mmu_cache() in various cases and in soem of 
those, the entry can be already present so for ARC700 cores we need to ensure that 
dups are not inserted !

>>   	/* Get free TLB slot: Set = computed from vaddr, way = random */
>>   	sr  TLBGetIndex, [ARC_REG_TLBCOMMAND]
>>   
>> @@ -287,6 +294,9 @@ ex_saved_reg1:
>>   #else
>>   	sr TLBInsertEntry, [ARC_REG_TLBCOMMAND]
>>   #endif
>> +#ifdef CONFIG_EZNPS_MTM_EXT
>> +88:
>> +#endif
> 
> Not sure if label itself required wrapping in ifdefs. It just makes code bulkier
> and harder to read.

I agree !

FWIW, after this patch, COMMIT_ENTRY_TO_MMU is totally unreadable - perhaps one of 
us needs to break it up into MMU ver specific implementations. But at any rate 
that can be after this patch.