read_cpuid_id() in arch/arm/kernel/setup.c
From: paul@pwsan.com (Paul Walmsley)
Date: 2015-03-16 16:54:23
Hi On Mon, 16 Mar 2015, Mason wrote:
On 15/03/2015 18:40, Mason wrote:quoted
On 13/03/2015 17:45, Russell King - ARM Linux wrote:quoted
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 <paul@pwsan.com> 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
quoted hunk ↗ jump to hunk
-- >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 <redacted> --- 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