From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Mon, 30 Oct 2017 13:12:51 +0000 Subject: [PATCH v3] arm64: prevent regressions in compressed kernel image size when upgrading to binutils 2.27 In-Reply-To: References: <20171027163341.57550-1-ndesaulniers@google.com> <20171030130834.GK4404@arm.com> Message-ID: <20171030131251.GM4404@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Mon, Oct 30, 2017 at 01:11:23PM +0000, Ard Biesheuvel wrote: > On 30 October 2017 at 13:08, Will Deacon wrote: > > On Fri, Oct 27, 2017 at 09:33:41AM -0700, Nick Desaulniers wrote: > >> Upon upgrading to binutils 2.27, we found that our lz4 and gzip > >> compressed kernel images were significantly larger, resulting is 10ms > >> boot time regressions. > >> > >> As noted by Rahul: > >> "aarch64 binaries uses RELA relocations, where each relocation entry > >> includes an addend value. This is similar to x86_64. On x86_64, the > >> addend values are also stored at the relocation offset for relative > >> relocations. This is an optimization: in the case where code does not > >> need to be relocated, the loader can simply skip processing relative > >> relocations. In binutils-2.25, both bfd and gold linkers did this for > >> x86_64, but only the gold linker did this for aarch64. The kernel build > >> here is using the bfd linker, which stored zeroes at the relocation > >> offsets for relative relocations. Since a set of zeroes compresses > >> better than a set of non-zero addend values, this behavior was resulting > >> in much better lz4 compression. > >> > >> The bfd linker in binutils-2.27 is now storing the actual addend values > >> at the relocation offsets. The behavior is now consistent with what it > >> does for x86_64 and what gold linker does for both architectures. The > >> change happened in this upstream commit: > >> https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=1f56df9d0d5ad89806c24e71f296576d82344613 > >> Since a bunch of zeroes got replaced by non-zero addend values, we see > >> the side effect of lz4 compressed image being a bit bigger. > >> > >> To get the old behavior from the bfd linker, "--no-apply-dynamic-relocs" > >> flag can be used: > >> $ LDFLAGS="--no-apply-dynamic-relocs" make > >> With this flag, the compressed image size is back to what it was with > >> binutils-2.25. > >> > >> If the kernel is using ASLR, there aren't additional runtime costs to > >> --no-apply-dynamic-relocs, as the relocations will need to be applied > >> again anyway after the kernel is relocated to a random address. > >> > >> If the kernel is not using ASLR, then presumably the current default > >> behavior of the linker is better. Since the static linker performed the > >> dynamic relocs, and the kernel is not moved to a different address at > >> load time, it can skip applying the relocations all over again." > > > > Do you have any numbers booting an uncompressed kernel Image without ASLR > > to see if skipping the relocs makes a measurable difference there? > > > > Do you mean built with ASLR support but executing at the offset it was > linked at? Yeah, sorry for being vague. Basically, the case where the relocs have all been resolved statically. In other words: what do we lose by disabling this optimisation? Will