From: Florian Weimer <fweimer@redhat.com>
To: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Cc: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
Wilco Dijkstra via Libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH 0/5] Added optimized memcpy/memmove/memset for A64FX
Date: Tue, 04 May 2021 12:17:36 +0200 [thread overview]
Message-ID: <87a6pan6pr.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20210504075643.GX9028@arm.com> (Szabolcs Nagy's message of "Tue, 4 May 2021 08:56:44 +0100")
* Szabolcs Nagy:
> The 04/30/2021 16:40, Wilco Dijkstra wrote:
>> >> Well it doesn't seem to behave like a NOP. So to avoid slowing down
>> >> all string functions, bti c must be removed completely, not just from
>> >> A64FX memcpy. Using a real NOP is fine in all cases as long as
>> >> HAVE_AARCH64_BTI is not defined.
>> >
>> > I'm probably confused, but: If BTI is active, many more glibc functions
>> > will have BTI markers. What makes the string functions special?
>>
>> Exactly. And at that point trying to remove it from memcpy is just pointless.
>>
>> The case we are discussing is where BTI is not turned on in GLIBC but we still
>> emit a BTI at the start of assembler functions for simplicity. By using a NOP
>> instead, A64FX will not execute BTI anywhere in GLIBC.
>
> the asm ENTRY was written with the assumption that bti c
> behaves like a nop when bti is disabled, so we don't have
> to make the asm conditional based on cflags.
>
> if that's not the case i agree with the patch, however we
> will have to review some other code (e.g. libgcc outline
> atomics asm) where we made the same assumption.
I find this discussion extremely worrisome. If bti c does not behave
like a nop, then we need a new AArch64 ABI variant to enable BTI.
That being said, a distribution with lots of bti c instructions in
binaries seems to run on A64FX CPUs, so I'm not sure what is going on.
Thanks,
Florian
next prev parent reply other threads:[~2021-05-04 10:17 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-12 12:52 Wilco Dijkstra
2021-04-12 18:53 ` Florian Weimer
2021-04-13 12:07 ` naohirot
2021-04-14 16:02 ` Wilco Dijkstra
2021-04-15 12:20 ` naohirot
2021-04-20 16:00 ` Wilco Dijkstra
2021-04-27 11:58 ` naohirot
2021-04-29 15:13 ` Wilco Dijkstra
2021-04-30 15:01 ` Szabolcs Nagy
2021-04-30 15:23 ` Wilco Dijkstra
2021-04-30 15:30 ` Florian Weimer
2021-04-30 15:40 ` Wilco Dijkstra
2021-05-04 7:56 ` Szabolcs Nagy
2021-05-04 10:17 ` Florian Weimer [this message]
2021-05-04 10:38 ` Wilco Dijkstra
2021-05-04 10:42 ` Szabolcs Nagy
2021-05-04 11:07 ` Florian Weimer
2021-05-06 10:01 ` naohirot
2021-05-06 14:26 ` Szabolcs Nagy
2021-05-06 15:09 ` Florian Weimer
2021-05-06 17:31 ` Wilco Dijkstra
2021-05-07 12:31 ` naohirot
2021-04-19 2:51 ` naohirot
2021-04-19 14:57 ` Wilco Dijkstra
2021-04-21 10:10 ` naohirot
2021-04-21 15:02 ` Wilco Dijkstra
2021-04-22 13:17 ` naohirot
2021-04-23 0:58 ` naohirot
2021-04-19 12:43 ` naohirot
2021-04-20 3:31 ` naohirot
2021-04-20 14:44 ` Wilco Dijkstra
2021-04-27 9:01 ` naohirot
2021-04-20 5:49 ` naohirot
2021-04-20 11:39 ` Wilco Dijkstra
2021-04-27 11:03 ` naohirot
2021-04-23 13:22 ` naohirot
-- strict thread matches above, loose matches on Subject: below --
2021-03-17 2:28 Naohiro Tamura
2021-03-29 12:03 ` Szabolcs Nagy
2021-05-10 1:45 ` naohirot
2021-05-14 13:35 ` Szabolcs Nagy
2021-05-19 0:11 ` naohirot
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=87a6pan6pr.fsf@oldenburg.str.redhat.com \
--to=fweimer@redhat.com \
--cc=Szabolcs.Nagy@arm.com \
--cc=Wilco.Dijkstra@arm.com \
--cc=libc-alpha@sourceware.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 read-only IMAP folder(s) and NNTP newsgroup(s).