Re: [PATCH v2 2/6] arm64: HWCAP: add support for AT_HWCAP2

From: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
To: Dave P Martin <Dave.Martin@arm.com>,
	Andrew Murray <Andrew.Murray@arm.com>
Cc: Catalin Marinas <Catalin.Marinas@arm.com>, nd <nd@arm.com>,
	Will Deacon <Will.Deacon@arm.com>,
	GNU C Library <libc-alpha@sourceware.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 2/6] arm64: HWCAP: add support for AT_HWCAP2
Date: Fri, 29 Mar 2019 16:44:05 +0000	[thread overview]
Message-ID: <83b59b0f-cee7-92d5-2ed2-5300f92d7329@arm.com> (raw)
In-Reply-To: <20190328112734.GZ3567@e103592.cambridge.arm.com>

On 28/03/2019 11:27, Dave Martin wrote:
> On Wed, Mar 27, 2019 at 03:24:15PM +0000, Andrew Murray wrote:
>> On Wed, Mar 27, 2019 at 03:02:25PM +0000, Andrew Murray wrote:
>>> I'll add documentation to Documentation/arm64 to indicate that the upper 32bits
>>> of AT_HWCAP will always be 0. Is this correct? 
>>
>> How about this (in Documentation/arm64/elf_hwcaps.txt)?
>> +
>> +
>> +4. Unused AT_HWCAP bits
>> +-----------------------
>> +
>> +Each AT_HWCAP and AT_HWCAP2 entry provides for up to 32 hwcaps contained
>> +in bits [31:0]. For interoperation with userspace we guarantee that the
>> +top bits [63:32] of AT_HWCAP will always be returned as 0.
> 
> Since the main reason for reserving bits [63:32] is ILP32, and it's
> still unclear when (or if) that will be merged, it feels a bit excessive
> to promise that we will never use these bits.
> 
> It sounds like glibc has a use for at most one bit in here.
> 
> So maybe we can reserve bit 63 (or 32, whatever) and promise that is
> zero, but leave the rest uncommitted for now.
> 
> Szabolcs, does that sound sensible?

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:

1288   /* The last entry in hwcap_extra is reserved for the "tls" pseudo-hwcap which
1289      indicates support for TLS.  This pseudo-hwcap is only used by old versions
1290      under which TLS support was optional.  The entry is no longer needed, but
1291      must remain for compatibility.  */
1292   hwcap_extra[63 - _DL_FIRST_EXTRA] = "tls";

https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/ldconfig.c;h=3bc9e618916ebb2fee29ffe3d114525a08390b43;hb=HEAD#l1288

and generic ld.so.cache handling code uses it:

https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/dl-cache.c;h=d8d1e2344e612d98689cf7d7ad965822d0ab6ed1;hb=HEAD#l265

since i don't understand how this tls bit was used
exactly i think it's better to use a different bit
for aarch64 ifunc abi hacks (e.g. 1ULL << 62)

cc += libc-alpha in case somebody knows more about
this bit.
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