From: mark.rutland@arm.com (Mark Rutland)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCHv2] ARM64: Add AT_ARM64_MIDR to the aux vector
Date: Tue, 1 Sep 2015 18:30:43 +0100 [thread overview]
Message-ID: <20150901173043.GE16430@leverpostej> (raw)
In-Reply-To: <4C8DD5E0-E1EA-40C6-B947-72189241023C@caviumnetworks.com>
[...]
> >>> On Sat, Aug 29, 2015 at 07:46:22PM +0100, Andrew Pinski wrote:
> >>> It is useful to pass down MIDR register down to userland if all of
> >>> the online cores are all the same type. This adds AT_ARM64_MIDR
> >>> aux vector type and passes down the midr system register.
> >>>
> >>> This is alternative to MIDR_EL1 part of
> >>> http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/358995.html.
> >>> It allows for faster access to midr_el1 than going through a trap and
> >>> does not exist if the set of cores are not the same.
> >>
> >> I'm not sure I follow the rationale. If speed is important the
> >> application can cache the value the first time it reads it with a trap.
> >
> > It is also about compatibility also. Exposing the register is not backwards compatible but using the aux vector is.
>
> That would also break big.little too. So either break it with hot plug or break it in userland, your choice.
The value wouldn't be representative of the system as a whole; that is
true. However, we never guaranteed that it was, while the aux vector
code implied that we did.
For optimisation that may be good enough; code optimized for a different
uArch should still function on another, even if it is slower.
> >> This also means that the behaviour is different across homogeneous and
> >> heterogeneous systems.
>
> That should be ok because it is still backwards compatible with what
> was done before. My goal here is just to allow quick easy access to
> midr in the case of a homogeneous system which I care about, thunderx
> and to allow glibc to select a memcpy/memset that is better for
> thunderx.
As I mentioned in the other thread, I think that HWCAP_CPUID is
sufficient to enable forwards and backwards compatibility. If it is
present then you can use the current CPU's MIDR to select a better
memcpy/memset if required.
Thanks,
Mark.
next prev parent reply other threads:[~2015-09-01 17:30 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-29 18:46 [PATCHv2] ARM64: Add AT_ARM64_MIDR to the aux vector Andrew Pinski
2015-09-01 16:33 ` Mark Rutland
2015-09-01 16:51 ` pinskia at gmail.com
2015-09-01 17:06 ` Pinski, Andrew
2015-09-01 17:30 ` Mark Rutland [this message]
2015-09-01 17:58 ` pinskia at gmail.com
2015-09-01 19:12 ` Siarhei Siamashka
2015-09-02 0:28 ` Pinski, Andrew
2015-09-02 13:57 ` Siarhei Siamashka
2015-09-02 14:52 ` Andrew Pinski
2015-09-02 17:11 ` Catalin Marinas
2015-09-02 17:21 ` Pinski, Andrew
2015-09-02 20:11 ` Thomas Gleixner
2015-09-03 17:14 ` Catalin Marinas
2015-09-01 17:19 ` Mark Rutland
2015-09-01 17:29 ` Pinski, Andrew
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150901173043.GE16430@leverpostej \
--to=mark.rutland@arm.com \
--cc=linux-arm-kernel@lists.infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).