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

[Christopher Covington]

On 02/01/2017 01:38 PM, Catalin Marinas wrote:
> On Wed, Feb 01, 2017 at 06:34:01PM +0000, Will Deacon wrote:
>> On Wed, Feb 01, 2017 at 06:22:44PM +0000, Catalin Marinas wrote:
>>> On Wed, Feb 01, 2017 at 05:59:48PM +0000, Will Deacon wrote:
>>>> On Wed, Feb 01, 2017 at 05:49:34PM +0000, Catalin Marinas wrote:
>>>>> On Wed, Feb 01, 2017 at 05:41:05PM +0000, Will Deacon wrote:
>>>>>> Maybe
>>>>>> just select ARM64_PAN if the erratum workaround is selected, then
>>>>>> runtime warning if we find that the h/w doesn't have PAN but does have
>>>>>> the erratum (which should never fire)?
>>>>>
>>>>> You still need this workaround even if you don't want any PAN (both sw
>>>>> and hw PAN disabled). I wouldn't want to select ARM64_PAN since it's not
>>>>> a dependency. It's more like if you do need a PAN, make sure you only
>>>>> use the hw one.
>>>>
>>>> True, in the case that all PAN options are disabled we still want this
>>>> to work. How about:
>>>>
>>>>   select ARM64_PAN if ARM64_SW_TTBR0_PAN
>>>
>>> As I replied to myself, the above would work for me as well, so let's go
>>> for this.
>>>
>>>> In fact, what's the reason for supporting SW_PAN and ARM64_PAN as a
>>>> config combination? Why not just have "PAN" that enables them both and
>>>> uses the hardware feature if it's there?
>>>
>>> Because SW PAN has a non-trivial performance hit. You would enable SW
>>> PAN only if you are paranoid about security. HW PAN, OTOH, is very cheap
>>> and I wouldn't want to miss enabling it in a single Image supporting
>>> ARMv8.0 and ARMv8.1 just because SW PAN is slow on ARMv8.0.
>>>
>>> IOW, ARM64_PAN is default y while ARM64_SW_TTBR0_PAN is default n.
>>
>> Ok, in that case, then how about another permutation: we make
>> ARM64_SW_TTBR0_PAN depend on ARM64_PAN? Then when you select "PAN Support"
>> you get a new menu option underneath it for the emulation? I think that
>> solves the erratum case and the use-case above.
> 
> The problem is that ARM64_PAN is an ARMv8.1 feature and currently
> grouped accordingly in Kconfig. ARM64_SW_TTBR0_PAN is complementary (and
> even not recommended on ARMv8.1). We can do this if we break the ARMv8.x
> feature grouping but just for this erratum, I don't think it's worth.

Thanks all. I've used "select ARM64_PAN if ARM64_SW_TTBR0_PAN" in v6.

Thanks,
Cov

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