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Re: [PATCH v2 2/6] arm64: HWCAP: add support for AT_HWCAP2
- From: Phil Blundell <pb at pbcl dot net>
- To: Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>, Dave P Martin <Dave dot Martin at arm dot com>, Andrew Murray <Andrew dot Murray at arm dot com>
- Cc: nd <nd at arm dot com>, Catalin Marinas <Catalin dot Marinas at arm dot com>, Will Deacon <Will dot Deacon at arm dot com>, "linux-arm-kernel at lists dot infradead dot org" <linux-arm-kernel at lists dot infradead dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 29 Mar 2019 16:57:46 +0000
- Subject: Re: [PATCH v2 2/6] arm64: HWCAP: add support for AT_HWCAP2
- References: <1550751657-30252-1-git-send-email-andrew.murray@arm.com> <1550751657-30252-3-git-send-email-andrew.murray@arm.com> <20190221184500.GO16031@e103592.cambridge.arm.com> <a3b8a7f1-983a-6d39-8100-88424dc05d6a@arm.com> <20190327150224.GE43527@e119886-lin.cambridge.arm.com> <20190327152414.GF43527@e119886-lin.cambridge.arm.com> <20190328112734.GZ3567@e103592.cambridge.arm.com> <83b59b0f-cee7-92d5-2ed2-5300f92d7329@arm.com>
On Fri, 2019-03-29 at 16:44 +0000, Szabolcs Nagy wrote:
> i think hwcap bit 63 is already reserved by glibc
> internally for tls support, it is not clear to me
> if that's still relevant (aarch64 post-dates tls
> support, so this might be historical cruft that can
> be cleaned up), i only see one comment about it:
At the time that bit was invented, it was only set when glibc was built
with TLS support enabled (i.e. USE_TLS defined). It would cause the
dynamic linker to add "tls" as a hwcap subdirectory in its search path, which I guess was useful in the days when TLS-enabled and non-TLS-enabled versions of the same DSO needed to be able to co-exist.
At some later date, I forget when, TLS became mandatory and the USE_TLS
condition went away. The comment seems to be saying that it was
thought necessary to keep the pseudo-hwcap bit set so that any DSOs in
a "tls" subdirectory would still be found. That seems like a debatable proposition even at the time, and even more so today, but I guess it was hard to prove the negative.
Even if aarch64 has always had TLS, in principle there might be some
ancient DSOs lurking in "tls" subdirectories which would no longer be
loaded if you were to stop setting that bit. I'm sure that would be manageable, but equally it doesn't seem that these bits are so scarce that it's worth going to any effort to scavenge old ones.
> i think it's better to use a different bit
> for aarch64 ifunc abi hacks (e.g. 1ULL << 62)
I agree.
p.