From mboxrd@z Thu Jan 1 00:00:00 1970 From: paul@pwsan.com (Paul Walmsley) Date: Mon, 16 Mar 2015 16:54:23 +0000 (UTC) Subject: read_cpuid_id() in arch/arm/kernel/setup.c In-Reply-To: <550697FE.1020804@free.fr> References: <55030A68.6070002@free.fr> <20150313161953.GS8656@n2100.arm.linux.org.uk> <550312BB.8020902@free.fr> <20150313164546.GV8656@n2100.arm.linux.org.uk> <5505C40C.1050001@free.fr> <550697FE.1020804@free.fr> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi On Mon, 16 Mar 2015, Mason wrote: > On 15/03/2015 18:40, Mason wrote: > > > On 13/03/2015 17:45, Russell King - ARM Linux wrote: > > > >> Yes, this one I like - and it probably fixes a potential latent bug > >> where the compiler was free to re-order that mrc outside of the if() > >> statement. > >> > >> Please wrap it up as a normal submission, thanks. > > > > Proposed patch at the end of this message. > > > > I'm now puzzling over why it's required to have "memory" > > in read_cpuid_ext's clobber list, and not in read_cpuid's? > > Same player shoot again. Reviewed-by: Paul Walmsley Looks reasonable to me. I'd suggest updating the patch message to describe your change, and why it's needed. Consider something like: --- Convert the open-coded MMFR0 register read in __get_cpu_architecture() to use the read_cpuid_ext() macro. This shortens the function and ensures that a memory clobber is used on the coprocessor read instruction. The memory clobber works around a bug in gcc 4.5. gcc 4.5 can reorder coprocessor read instructions with respect to other code, disregarding potential side-effects of the coprocessor read. --- Once you've got something that you're happy with, and have reposted it to the public lists, I believe the next step will be for you to post it to rmk's patch tracker at: http://www.arm.linux.org.uk/developer/patches/ - Paul > > -- >8 -- > Date: Sun, 15 Mar 2015 17:59:53 +0100 > Subject: [PATCH] Use read_cpuid_ext() macro instead of inline asm > > In commit 067e710b9a98 ("ARM: 7801/1: prevent gcc 4.5 from reordering extended > CP15 reads above is_smp() test") Paul Walmsley fixed read_cpuid_ext() and added > the following comment. > > The memory clobber prevents gcc 4.5 from reordering the mrc before > any is_smp() tests, which can cause undefined instruction aborts on > ARM1136 r0 due to the missing extended CP15 registers. > > Signed-off-by: Mason > --- > arch/arm/kernel/setup.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c > index e55408e..1d60beb 100644 > --- a/arch/arm/kernel/setup.c > +++ b/arch/arm/kernel/setup.c > @@ -246,12 +246,9 @@ static int __get_cpu_architecture(void) > if (cpu_arch) > cpu_arch += CPU_ARCH_ARMv3; > } else if ((read_cpuid_id() & 0x000f0000) == 0x000f0000) { > - unsigned int mmfr0; > - > /* Revised CPUID format. Read the Memory Model Feature > * Register 0 and check for VMSAv7 or PMSAv7 */ > - asm("mrc p15, 0, %0, c0, c1, 4" > - : "=r" (mmfr0)); > + unsigned int mmfr0 = read_cpuid_ext(CPUID_EXT_MMFR0); > if ((mmfr0 & 0x0000000f) >= 0x00000003 || > (mmfr0 & 0x000000f0) >= 0x00000030) > cpu_arch = CPU_ARCH_ARMv7; > -- > 2.3.2 > - Paul