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

[Christopher Covington]

On 01/31/2017 12:56 PM, Marc Zyngier wrote:
> On 31/01/17 17:48, Christopher Covington wrote:
>> On 01/31/2017 07:37 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.
>>>
>>> Based on my understanding that entries allocated to the reserved ASID
>>> will not be used for subsequent page table walks (and so we don't have
>>> asynchronous behaviour to contend with), this sounds fine to me.
>>>
>>> Thanks for taking the time to clarify the details on that.
>>>
>>>> Based on work by Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
>>>>
>>>> Signed-off-by: Christopher Covington <cov@xxxxxxxxxxxxxx>
>>>> ---
>>>>  Documentation/arm64/silicon-errata.txt |  1 +
>>>>  arch/arm64/Kconfig                     | 11 +++++++++++
>>>>  arch/arm64/include/asm/assembler.h     | 23 +++++++++++++++++++++++
>>>>  arch/arm64/include/asm/cpucaps.h       |  3 ++-
>>>>  arch/arm64/include/asm/mmu_context.h   |  8 +++++++-
>>>>  arch/arm64/kernel/cpu_errata.c         |  7 +++++++
>>>>  arch/arm64/mm/context.c                | 11 +++++++++++
>>>>  arch/arm64/mm/proc.S                   |  1 +
>>>>  8 files changed, 63 insertions(+), 2 deletions(-)
>>>
>>> Don't we need to use pre_ttbr0_update_workaround in <asm/asm-uaccess.h>
>>> for CONFIG_ARM64_SW_TTBR0_PAN? We implicitly switch to the reserved ASID
>>> for the empty table in __uaccess_ttbr0_disable.
>>>
>>> That also means we have to invalidate the reserved ASID so as to not
>>> accidentally hit while uaccess is disabled.
>>
>> The CPU in question (Falkor v1) has hardware PAN support. Do we need
>> to worry about including the workaround in the SW PAN code in that case?
> 
> Given that all ARMv8 CPUs can support SW_PAN, it is more likely to be
> enabled than the ARMv8.1 PAN. I'd vote for supporting the workaround in
> that case too, and hope that people do enable the HW version.

Okay, I'll do my best to add support for the SW PAN case. I rebased and
submitted v6 of the E1009 patch [1] so that it no longer depends on this
patch landing first, if you all are inclined to pick it up while work on
this E1003 patch continues.

1. https://patchwork.kernel.org/patch/9547923/

Thanks,
Christopher

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