Re: [PATCH v2 0/5] arm64: advertise availability of CRC and crypto instructions

From: Ard Biesheuvel
Date: Mon Jan 20 2014 - 14:02:17 EST

On 20 January 2014 19:55, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
> On Mon, 20 Jan 2014, Ard Biesheuvel wrote:
>> On 20 January 2014 19:17, Nicolas Pitre <nicolas.pitre@xxxxxxxxxx> wrote:
>> > On Mon, 20 Jan 2014, Ard Biesheuvel wrote:
>> >> Calling getauxval(AT_HWCAP) on an outdated will give you the
>> >> whole value, not just the bits whose meaning was known to glibc at the
>> >> time.
>> >> So if desired, a program can interpret AT_HWCAP itself.
>> >>
>> >> AT_HWCAP2 is completely new, so you won't be able to retrieve it using
>> >> getauxval() on an older libc.
>> >
>> > I agree. And I don't dispute the bit placement choice either.
>> >
>> > Still, an old glibc calling getauxval(AT_HWCAP) should already be
>> > prepared to receive and rightfully ignore those bits it didn't know the
>> > meaning of at the time. So "preserving some future extensions in HWCAP
>> > for older glibc" as a justification makes little sense to me... unless
>> > I'm missing something?
>> >
>> > Even if applications interpret those bits themselves, supposing they
>> > still need to be linked against an old glibc, then why would
>> > yet-to-be-defined future extensions be more important to be signaled
>> > using the lower 32 bits than the extensions you propose? That is what I
>> > don't get.
>> >
>> In the general case, you are quite right.
>> In this particular case, the extensions for which I am adding the
>> feature bits are not supported on any hardware currently known or
>> supported by the ARM port. At this time, the only known CPUs
>> supporting these extensions are ARMv8 CPUs executing in 32-bit
>> compatibility mode (i.e., ARMv8 defines instructions for the 32-bit
>> execution state using previously unallocated opcodes)
> So?
>> So the only reason (currently) for adding these feature bits to the
>> ARM port is to align with the ARMv8 32-bit compat mode so that a
>> 32-bit userland requires no knowledge about whether its 32-bit
>> execution environment is hosted by an ARM or an arm64 kernel. In the
>> future, ARMv8 32-bit only CPUs may well turn up that support these
>> extensions as well.
> I agree with all you've said so far. But that doesn't answer my
> question.
> And my unanswered question isn't that important either.

Quoting Russell:

On 18 December 2013 12:42, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxxx> wrote:
> The point is that they'll never appear on an ARMv7 implementation because
> they're not part of the ARMv7 architecture. I see no point in needlessly
> polluting ARM32 with ARM64 stuff - in exactly the same way that you see
> no point in polluting ARM64 with ARM32 stuff.
> So, frankly, find a different way to this. We don't need to needlessly
> waste HWCAP bits on ARM32.

So my idea was to use HWCAP2 bits instead ...

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