Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

[Ryan Roberts <ryan.roberts@arm.com>]

On 13/03/2025 18:36, Marc Zyngier wrote:
> On Thu, 13 Mar 2025 18:22:00 +0000,
> Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 13/03/2025 17:34, Marc Zyngier wrote:
>>> On Thu, 13 Mar 2025 10:41:10 +0000,
>>> Mikołaj Lenczewski <miko.lenczewski@arm.com> wrote:
>>>>
>>>> diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
>>>> index c6b185b885f7..9728faa10390 100644
>>>> --- a/arch/arm64/kernel/pi/idreg-override.c
>>>> +++ b/arch/arm64/kernel/pi/idreg-override.c
>>>> @@ -209,6 +209,7 @@ static const struct ftr_set_desc sw_features __prel64_initconst = {
>>>>  		FIELD("nokaslr", ARM64_SW_FEATURE_OVERRIDE_NOKASLR, NULL),
>>>>  		FIELD("hvhe", ARM64_SW_FEATURE_OVERRIDE_HVHE, hvhe_filter),
>>>>  		FIELD("rodataoff", ARM64_SW_FEATURE_OVERRIDE_RODATA_OFF, NULL),
>>>> +		FIELD("nobbml2", ARM64_SW_FEATURE_OVERRIDE_NOBBML2, NULL),
>>>>  		{}
>>>>  	},
>>>>  };
>>>> @@ -246,6 +247,7 @@ static const struct {
>>>>  	{ "rodata=off",			"arm64_sw.rodataoff=1" },
>>>>  	{ "arm64.nolva",		"id_aa64mmfr2.varange=0" },
>>>>  	{ "arm64.no32bit_el0",		"id_aa64pfr0.el0=1" },
>>>> +	{ "arm64.nobbml2",		"arm64_sw.nobbml2=1" },
>>>
>>> Why is that a SW feature? This looks very much like a HW feature to
>>> me, and you should instead mask out ID_AA64MMFR2_EL1.BBM, and be done
>>> with it. Something like:
>>
>> I think this implies that we would expect the BBM field to be advertising BBML2
>> support normally and we would check for that as part of the cpufeature
>> detection. That's how Miko was doing it in v2, but Yang pointed out that
>> AmpereOne, which supports BBML2+NOABORT semantics, doesn't actually advertise
>> BBML2 in its MMFR2. So we don't want to check that field, and instead rely
>> solely on the MIDR allow-list + a command line override. It was me that
>> suggested putting that in the SW feature register, and I think that still sounds
>> like the right solution for this situation?
> 
> I think this is mixing two different things:
> 
> - preventing BBM-L2 from being visible to the kernel: this is what my
>   suggestion is doing by nuking an architectural feature in the
>   relevant register
> 
> - random HW not correctly advertising what they are doing: this is an
>   erratum workaround
> 
> I'd rather we don't conflate the two things, and make them very
> explicitly distinct.

It all sounds so obvious when you put it like that! :)

I'm guessing there is a layer where the workaround can be applied to the
sanitised feature registers on a per-cpu basis and that won't affect this global
override which will remain as an overlay on top? If so then that sounds perfect
(you can probably tell I find the whole feature management framework rather
inpeneterable). That workaround would be added as part of Yang's series anyway.

So sounds like we are back to testing MMFR2.BBM in the matches function, with
the addition of Maz's proposal above. Sorry for sending you round the houses, Miko.

Thanks,
Ryan

> 
> Thanks,
> 
> 	M.
>