From: Jiaxun Yang <jiaxun.yang@flygoat.com>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc: YunQiang Su <yunqiang.su@cipunited.com>,
Richard Sandiford <richard.sandiford@arm.com>,
gcc-patches@gcc.gnu.org, YunQiang Su <syq@debian.org>
Subject: Re: [PATCH v2] MIPS: add speculation_barrier support
Date: Sun, 7 May 2023 19:47:14 +0100 [thread overview]
Message-ID: <EC557C8F-4487-456F-9BD1-ACE9EBB7453A@flygoat.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2305041628470.42277@angie.orcam.me.uk>
> 2023年5月7日 18:34,Maciej W. Rozycki <macro@orcam.me.uk> 写道:
>
> On Wed, 3 May 2023, Jiaxun Yang wrote:
>
>> Since it’s possible to run R2- binary on R2+ processor, we’d better find a
>> semantic that do eliminate speculation on all processors. While SSNOPs
>> on R2+ processors is pretty much undefined, there is no guarantee that
>> SSNOP sequence can eliminate speculation.
>
> Not exactly undefined on R2+, SSNOP is still required to single-issue, so
> it does act as an execution barrier. Good point otherwise.
>
> Both EHB and J[AL]R.HB are backwards compatible however (except for an
> obscure 4Kc J[AL]R.HB erratum I came across once and which may be no
> longer relevant), so I think the legacy sequence ought to just return via
> JR.HB as well, therefore providing the required semantics with newer
> hardware. If it does trap for 4Kc, then the OS can emulate it (and we can
> ignore it for bare metal, deferring to whoever might be interested for a
> workaround).
Hmm, I just checked MIPS-IV manual, it seems like HB bit (bit 10) is defined as
zero for both JR and JALR.
Is it actually omitted in implementation?
Thanks
Jiaxun
next prev parent reply other threads:[~2023-05-07 18:47 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-28 12:33 [PATCH] " YunQiang Su
2023-04-28 12:36 ` Jiaxun Yang
2023-04-28 13:07 ` YunQiang Su
2023-04-28 13:12 ` [PATCH v2] " YunQiang Su
2023-05-03 18:29 ` Richard Sandiford
2023-05-03 21:04 ` Maciej W. Rozycki
2023-05-03 22:12 ` Jiaxun Yang
2023-05-07 17:34 ` Maciej W. Rozycki
2023-05-07 18:47 ` Jiaxun Yang [this message]
2023-05-07 19:16 ` Maciej W. Rozycki
2023-05-12 10:03 ` [PATCH v3] " YunQiang Su
2023-05-12 10:30 ` [PATCH v4] " YunQiang Su
2023-05-31 9:43 ` YunQiang Su
2023-05-31 10:35 ` Maciej W. Rozycki
2023-06-01 4:26 ` [PATCH v5] MIPS: Add " YunQiang Su
2023-06-08 12:35 ` Richard Earnshaw (lists)
2023-06-16 7:53 ` YunQiang Su
2023-06-16 8:38 ` Xi Ruoyao
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