linux-kernel - Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

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Date:   Fri, 27 Jan 2017 16:52:23 -0500
From:   Christopher Covington <cov@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, shankerd@...eaurora.org,
        timur@...eaurora.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, Jon Masters <jcm@...hat.com>,
        Neil Leeder <nleeder@...eaurora.org>,
        Mark Langsdorf <mlangsdo@...hat.com>
Subject: Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

Hi Mark,

On 01/27/2017 09:38 AM, Mark Rutland wrote:
> On Wed, Jan 25, 2017 at 10:52:30AM -0500, Christopher Covington wrote:
>> The Qualcomm Datacenter Technologies Falkor v1 CPU may allocate TLB entries
>> using an incorrect ASID when TTBRx_EL1 is being updated. When the erratum
>> is triggered, page table entries using the new translation table base
>> address (BADDR) will be allocated into the TLB using the old ASID. All
>> circumstances leading to the incorrect ASID being cached in the TLB arise
>> when software writes TTBRx_EL1[ASID] and TTBRx_EL1[BADDR], a memory
>> operation is in the process of performing a translation using the specific
>> TTBRx_EL1 being written, and the memory operation uses a translation table
>> descriptor designated as non-global. EL2 and EL3 code changing the EL1&0
>> ASID is not subject to this erratum because hardware is prohibited from
>> performing translations from an out-of-context translation regime.
>>
>> Consider the following pseudo code.
>>
>>   write new BADDR and ASID values to TTBRx_EL1
>>
>> Replacing the above sequence with the one below will ensure that no TLB
>> entries with an incorrect ASID are used by software.
>>
>>   write reserved value to TTBRx_EL1[ASID]
>>   ISB
>>   write new value to TTBRx_EL1[BADDR]
>>   ISB
>>   write new value to TTBRx_EL1[ASID]
>>   ISB
>>
>> When the above sequence is used, page table entries using the new BADDR
>> value may still be incorrectly allocated into the TLB using the reserved
>> ASID. Yet this will not reduce functionality, since TLB entries incorrectly
>> tagged with the reserved ASID will never be hit by a later instruction.
> 
> I agree that there should be no explicit accesses to the VAs for these
> entries. So tasks should not see erroneous VAs, and we shouldn't see
> synchronous TLB conflict aborts.
> 
> Regardless, can this allow conflicting TLB entries to be allocated to
> the reserved ASID? e.g. if one task has a 4K mapping at a given VA, and
> another has a 2M mapping which covers that VA, can both be allocated
> into the TLBs under the reserved ASID?
> 
> Can that have any effect on asynchronous TLB lookups or page table
> walks, e.g. for speculated accesses?

A speculative access that inserts an entry into the TLB could
possibly find the conflict but will not signal it. Does that answer
your question?

Thanks,
Cov

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