linux-kernel - [PATCH v2] uapi/auxvec: Define AT_HWCAP3 and AT

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Date: Wed, 14 Feb 2024 16:34:06 -0600
From: Peter Bergner <bergner@...ux.ibm.com>
To: linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Cc: Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        Nick Piggin <npiggin@....ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>, Arnd Bergmann <arnd@...nel.org>
Subject: [PATCH v2] uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux vector,
 entries

Changes from v1:
- Add Acked-by lines.

The powerpc toolchain keeps a copy of the HWCAP bit masks in our TCB for fast
access by the __builtin_cpu_supports built-in function.  The TCB space for
the HWCAP entries - which are created in pairs - is an ABI extension, so
waiting to create the space for HWCAP3 and HWCAP4 until we need them is
problematical.  Define AT_HWCAP3 and AT_HWCAP4 in the generic uapi header
so they can be used in glibc to reserve space in the powerpc TCB for their
future use.

I scanned through the Linux and GLIBC source codes looking for unused AT_*
values and 29 and 30 did not seem to be used, so they are what I went
with.  This has received Acked-by's from both GLIBC and Linux kernel
developers and no reservations or Nacks from anyone.

Arnd, we seem to have consensus on the patch below.  Is this something
you could take and apply to your tree? 

Peter

Signed-off-by: Peter Bergner <bergner@...ux.ibm.com>
Acked-by: Adhemerval Zanella <adhemerval.zanella@...aro.org> (glibc)
Acked-by: Nicholas Piggin <npiggin@...il.com> (powerpc)
Acked-by: Szabolcs Nagy <szabolcs.nagy@....com> (arm)

---
 include/uapi/linux/auxvec.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
index 6991c4b8ab18..cc61cb9b3e9a 100644
--- a/include/uapi/linux/auxvec.h
+++ b/include/uapi/linux/auxvec.h
@@ -32,6 +32,8 @@
 #define AT_HWCAP2 26	/* extension of AT_HWCAP */
 #define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
 #define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
+#define AT_HWCAP3 29	/* extension of AT_HWCAP */
+#define AT_HWCAP4 30	/* extension of AT_HWCAP */

 #define AT_EXECFN  31	/* filename of program */

-- 
2.39.3