[PATCH 7/8] ARM: Add hwcap for Speculation Barrier(SB)

[Robin Murphy]

On 2022-11-16 14:06, Linus Walleij wrote:
> On Wed, Oct 26, 2022 at 7:53 AM Amit Daniel Kachhap
> <amit.kachhap at arm.com> wrote:
> 
>> Speculation Barrier(FEAT_SB) is a feature present in AArch32 state for
>> Armv8 and is represented by ISAR6.SB identification register.
>>
>> This feature denotes the presence of SB instruction and hence adding a
>> hwcap will enable the userspace to check it before trying to use this
>> instruction.
>>
>> This commit adds the ID feature bit detection, and uses elf_hwcap2
>> accordingly.
>>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap at arm.com>
> 
> The patch is fine, the following is a question.
> 
> I see that the aarch64 kernel is using this instruction in the kernel
> for speculation barriers, and after this the aarch32 userspace can
> use it too.
> 
> Does it make sense to ask the question whether this could be
> compiled into and used by a aarch32 kernel, provided it is
> configured for a core known to support it? (This is assuming that
> the compiler also knows about it.)
> 
> I'm asking because speculation barriers is something we have a
> lot of due to the constant security problems so it might be something
> handy to have in the toolbox.
> 
> It might be just adding too much complexity... I know.

Certainly none of Arm's CPUs new enough to implement FEAT_SB still 
support AArch32 at EL1.

Robin.