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From: Kees Cook <keescook@chromium.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	linux-efi@vger.kernel.org,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Manoj Gupta" <manojgupta@google.com>,
	"Will Deacon" <will@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-arch@vger.kernel.org, "Andi Kleen" <ak@linux.intel.com>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	x86@kernel.org, "Russell King" <linux@armlinux.org.uk>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	clang-built-linux@googlegroups.com,
	"Ingo Molnar" <mingo@redhat.com>,
	"Luis Lozano" <llozano@google.com>,
	"Borislav Petkov" <bp@suse.de>, "Arnd Bergmann" <arnd@arndb.de>,
	"Jian Cai" <jiancai@google.com>,
	"Nathan Chancellor" <natechancellor@gmail.com>,
	"Peter Collingbourne" <pcc@google.com>,
	linux-arm-kernel@lists.infradead.org,
	"Michal Marek" <michal.lkml@markovi.net>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	"James Morse" <james.morse@arm.com>
Subject: Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections
Date: Fri, 21 Aug 2020 12:18:10 -0700
Message-ID: <202008211216.3812BBA0C8@keescook> (raw)
In-Reply-To: <20200804160649.GA2409491@rani.riverdale.lan>

On Tue, Aug 04, 2020 at 12:06:49PM -0400, Arvind Sankar wrote:
> On Mon, Aug 03, 2020 at 09:45:32PM -0700, Andi Kleen wrote:
> > > Why is that? Both .text and .text.hot have alignment of 2^4 (default
> > > function alignment on x86) by default, so it doesn't seem like it should
> > > matter for packing density.  Avoiding interspersing cold text among
> > 
> > You may lose part of a cache line on each unit boundary. Linux has 
> > a lot of units, some of them small. All these bytes add up.
> 
> Separating out .text.unlikely, which isn't aligned, slightly _reduces_
> this loss, but not by much -- just over 1K on a defconfig. More
> importantly, it moves cold code out of line (~320k on a defconfig),
> giving better code density for the hot code.
> 
> For .text and .text.hot, you lose the alignment padding on every
> function boundary, not unit boundary, because of the 16-byte alignment.
> Whether .text.hot and .text are arranged by translation unit or not
> makes no difference.
> 
> With *(.text.hot) *(.text) you get HHTT, with *(.text.hot .text) you get
> HTHT, but in both cases the individual chunks are already aligned to 16
> bytes. If .text.hot _had_ different alignment requirements to .text, the
> HHTT should actually give better packing in general, I think.

Okay, so at the end of the conversation, I think it looks like this
patch is correct: it collects the hot, unlikely, etc into their own
areas (e.g. HHTTUU is more correct than HTUHTU), so this patch stands
as-is.

-- 
Kees Cook

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Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-31 23:07 [PATCH v5 00/36] Warn on orphan section placement Kees Cook
2020-07-31 23:07 ` [PATCH v5 01/36] x86/boot/compressed: Move .got.plt entries out of the .got section Kees Cook
2020-07-31 23:07 ` [PATCH v5 02/36] x86/boot/compressed: Force hidden visibility for all symbol references Kees Cook
2020-07-31 23:07 ` [PATCH v5 03/36] x86/boot/compressed: Get rid of GOT fixup code Kees Cook
2020-07-31 23:07 ` [PATCH v5 04/36] x86/boot: Add .text.* to setup.ld Kees Cook
2020-07-31 23:07 ` [PATCH v5 05/36] x86/boot: Remove run-time relocations from .head.text code Kees Cook
2020-07-31 23:42   ` Nick Desaulniers
2020-07-31 23:07 ` [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32, 64}.S Kees Cook
2020-08-07 18:12   ` Nick Desaulniers
2020-08-07 20:20     ` [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 07/36] x86/boot: Check that there are no run-time relocations Kees Cook
2020-07-31 23:07 ` [PATCH v5 08/36] vmlinux.lds.h: Create COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 09/36] vmlinux.lds.h: Add .gnu.version* to COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 10/36] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook
2020-07-31 23:07 ` [PATCH v5 11/36] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook
2020-07-31 23:07 ` [PATCH v5 12/36] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook
2020-07-31 23:07 ` [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections Kees Cook
2020-08-01  3:51   ` Arvind Sankar
2020-08-01  6:18     ` Kees Cook
2020-08-01 17:27       ` Arvind Sankar
2020-08-03 19:05     ` Andi Kleen
2020-08-03 20:15       ` Arvind Sankar
2020-08-04  1:19         ` Fāng-ruì Sòng
2020-08-04  4:45         ` Andi Kleen
2020-08-04  5:32           ` Fāng-ruì Sòng
2020-08-04 16:06           ` Arvind Sankar
2020-08-21 19:18             ` Kees Cook [this message]
2020-07-31 23:07 ` [PATCH v5 14/36] efi/libstub: Disable -mbranch-protection Kees Cook
2020-07-31 23:07 ` [PATCH v5 15/36] arm64/mm: Remove needless section quotes Kees Cook
2020-07-31 23:08 ` [PATCH v5 16/36] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook
2020-07-31 23:08 ` [PATCH v5 17/36] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook
2020-07-31 23:08 ` [PATCH v5 18/36] arm64/build: Use common DISCARDS in linker script Kees Cook
2020-07-31 23:08 ` [PATCH v5 19/36] arm64/build: Add missing DWARF sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 20/36] arm64/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 21/36] arm64/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 22/36] arm/build: Refactor linker script headers Kees Cook
2020-07-31 23:08 ` [PATCH v5 23/36] arm/build: Explicitly keep .ARM.attributes sections Kees Cook
2020-08-03 19:02   ` Nick Desaulniers
2020-08-17 22:06     ` Fangrui Song
2020-07-31 23:08 ` [PATCH v5 24/36] arm/build: Add missing sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 25/36] arm/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 26/36] arm/boot: Handle all sections explicitly Kees Cook
2020-07-31 23:08 ` [PATCH v5 27/36] arm/boot: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 28/36] x86/asm: Avoid generating unused kprobe sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section Kees Cook
2020-08-01  2:12   ` Arvind Sankar
2020-08-01  5:32     ` Kees Cook
2020-08-21 17:49     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 30/36] x86/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 31/36] x86/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts Kees Cook
2020-08-01  1:47   ` Arvind Sankar
2020-08-01  2:53     ` Arvind Sankar
2020-08-01  5:36       ` Kees Cook
2020-08-01 17:12         ` Arvind Sankar
2020-08-21 18:24           ` Kees Cook
2020-08-01  5:35     ` Kees Cook
2020-08-01 17:00       ` Arvind Sankar
2020-08-21 18:19     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 33/36] x86/boot/compressed: Remove, discard, or assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 34/36] x86/boot/compressed: Add missing debugging sections to output Kees Cook
2020-07-31 23:08 ` [PATCH v5 35/36] x86/boot/compressed: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 36/36] arm/build: Assert for unwanted sections Kees Cook

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