From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Steven Rostedt <rostedt@goodmis.org>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
Andy Lutomirski <luto@amacapital.net>,
Oleg Nesterov <oleg@redhat.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Alexei Starovoitov <ast@plumgrid.com>,
Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions
Date: Wed, 1 Apr 2015 10:29:35 +0200 [thread overview]
Message-ID: <20150401082934.GA23533@gmail.com> (raw)
In-Reply-To: <1427821211-25099-7-git-send-email-dvlasenk@redhat.com>
* Denys Vlasenko <dvlasenk@redhat.com> wrote:
> After TESTs, use logically correct JZ mnemonic instead of JE
> (this doesn't change code).
>
> Tidy up CMPW insns:
>
> Modern CPUs are not good with 16-bit operations.
> The instructions with 16-bit immediates are especially bad,
> on many CPUs they cause length changing prefix stall
> in the decoders, costing ~6 cycles to recover.
>
> Replace CMPWs with CMPLs.
> Of these, for form with 8-bit sign-extended immediates
> it is a win because they are smaller now
> (no 0x66 prefix anymore);
> ones with 16-bit immediates are faster.
This patch does JE->JZ transitions, but it also does CMPW instruction
tweaking - which was buggy as Brian (miraculously!) noticed.
This isn't the first such incident, and I made this point about three
times already in the past, but it appears I've not made it loud
enough: which part of 'do not put two unrelated changes into the same
patch' did you not understand??
We _DO NOT PUT_ multiple, unrelated changes to assembly files into a
single patch! And we _especially_ don't mix them up under a
meaningless, repetitive, misleading 'tidy up instructions' title!
Full stop.
The titles of the two patches should have been something like:
x86/asm/entry/32: Convert JNE to JNZ mnemonics, to improve readability
x86/asm/entry/32: Optimize CMPW to CMPL instructions, to make use of automatic zero-extend
We were lucky that Brian was alert enough to have read through a
misleadingly titled, seemingly harmless patch and noticed the bug in
your patch, but heck you made it hard!!!
And no, it's not a problem if you create a dozen trivial looking
patches and have to wait a bit more for them to trickle into the
maintainer tree: asm patches are seldom trivial, and even if they are
trivial, both reviewability and bisectability will improve from the
process.
You are doing a nice job improving the x86/asm/entry code, but if you
cannot create suitably conservative, maximally reviewable and
maximally bisectable patches to x86/asm then I won't be able to apply
assembly patches from you!
</rant>
Thanks,
Ingo
next prev parent reply other threads:[~2015-04-01 8:29 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-31 17:00 [PATCH 1/9] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path Denys Vlasenko
2015-03-31 17:00 ` [PATCH 2/9] x86/asm/entry/32: Use PUSH instructions to build pt_regs on stack Denys Vlasenko
2015-04-01 8:51 ` Ingo Molnar
2015-04-01 13:12 ` Denys Vlasenko
2015-04-01 13:21 ` Ingo Molnar
2015-04-01 13:53 ` Borislav Petkov
2015-04-02 12:25 ` [tip:x86/asm] x86/asm/entry/32: Use smaller PUSH instructions instead of MOV, to build 'pt_regs' " tip-bot for Denys Vlasenko
2015-03-31 17:00 ` [PATCH 3/9] x86/asm/entry/64: simplify retint_kernel label usage, make retint_restore_args label local Denys Vlasenko
2015-04-02 12:25 ` [tip:x86/asm] x86/asm/entry/64: Simplify " tip-bot for Denys Vlasenko
2015-03-31 17:00 ` [PATCH 4/9] x86/asm/entry/64: remove redundant DISABLE_INTERRUPTS Denys Vlasenko
2015-04-02 12:25 ` [tip:x86/asm] x86/asm/entry/64: Remove redundant DISABLE_INTERRUPTS() tip-bot for Denys Vlasenko
2015-03-31 17:00 ` [PATCH 5/9] x86/asm/entry/64: simplify looping around preempt_schedule_irq Denys Vlasenko
2015-04-02 12:26 ` [tip:x86/asm] x86/asm/entry/64: Simplify looping around preempt_schedule_irq() tip-bot for Denys Vlasenko
2015-03-31 17:00 ` [PATCH 6/9] x86/asm/entry/64: tidy up some instructions Denys Vlasenko
2015-03-31 17:00 ` [PATCH 7/9] x86/asm/entry/32: " Denys Vlasenko
2015-03-31 22:21 ` Brian Gerst
2015-03-31 23:09 ` Linus Torvalds
2015-04-01 11:10 ` Denys Vlasenko
2015-04-01 15:50 ` Linus Torvalds
2015-04-01 20:52 ` Denys Vlasenko
2015-04-01 20:57 ` H. Peter Anvin
2015-04-01 22:14 ` Linus Torvalds
2015-04-02 0:32 ` Brian Gerst
2015-04-01 8:29 ` Ingo Molnar [this message]
2015-03-31 17:00 ` [PATCH 8/9] x86/asm: replace MOVQ $imm,%reg with MOVL Denys Vlasenko
2015-04-02 12:26 ` [tip:x86/asm] x86/asm: Replace "MOVQ $imm, %reg" " tip-bot for Denys Vlasenko
2015-03-31 17:00 ` [PATCH 9/9] x86/asm/entry/64: use local label to skip around sycall dispatch Denys Vlasenko
2015-04-02 12:26 ` [tip:x86/asm] x86/asm/entry/64: Use " tip-bot for Denys Vlasenko
2015-04-02 12:25 ` [tip:x86/asm] x86/asm/entry/64: Do not TRACE_IRQS fast SYSRET64 path tip-bot for Denys Vlasenko
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