Messages in this thread | | | Date | Mon, 20 Jan 2014 13:55:07 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH v2 0/5] arm64: advertise availability of CRC and crypto instructions |
| |
On Mon, 20 Jan 2014, Ard Biesheuvel wrote:
> On 20 January 2014 19:17, Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > > On Mon, 20 Jan 2014, Ard Biesheuvel wrote: > >> Calling getauxval(AT_HWCAP) on an outdated libc.so will give you the 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.
Nicolas
| |