[PATCH] Disable ARMv6 memory barriers on ARMv5 processors

[David Holmes]

Hi Jakub,

This patch seems reasonable to me. I've cc'd Boris at BellSoft to make 
sure he sees this.

Thanks,
David

On 13/11/2018 8:33 AM, Jakub Vaněk wrote:
> Hi,
> 
> This patch disables ARMv6+ memory barriers on older ARM processors.
> This is implemented by adding a check to the non-ARMv7 code path if it
> is an ARMv6. When this extra check is not performed, the code crashes
> with illegal instruction. On ARMv5, I believe these memory barriers
> are not necessary, because there is always only one processor.
> 
> We know that processors older than ARMv7 are not supported in OpenJDK.
> However, we are so far successfully patching new OpenJDK to run on the
> Lego Mindstorms EV3 brick (ARM926EJ-S CPU). The project can be found
> at https://github.com/ev3dev-lang-java/openjdk-ev3 . We want to
> minimize the number of maintained patches, so we want to upstream
> whatever is possible.
> 
> This patch was built many times as part of our CI jobs hosted on Adopt
> OpenJDK infrastructure: https://ci.adoptopenjdk.net/view/ev3dev/ We
> have not yet run full jtreg tests on the target platform, but Juan
> Antonio Brena Moral invested his time into Adopt's openjdktests in
> QEMU.
> 
> Regards,
> 
> Jakub Vanek
> 
> # HG changeset patch
> # User Jakub Vaněk <linuxtardis at gmail.com>
> # Date 1542059840 -3600
> #      Mon Nov 12 22:57:20 2018 +0100
> # Node ID 97525849d04b8e911bf592dd03d3518dd33bc552
> # Parent  a609d549992a4d094a514a6c0f5b79443192d8bf
> Disable ARMv6 memory barriers on ARMv5 processors
> 
> diff --git a/src/hotspot/cpu/arm/assembler_arm_32.hpp
> b/src/hotspot/cpu/arm/assembler_arm_32.hpp
> --- a/src/hotspot/cpu/arm/assembler_arm_32.hpp
> +++ b/src/hotspot/cpu/arm/assembler_arm_32.hpp
> @@ -498,7 +498,7 @@
>     void dmb(DMB_Opt opt, Register reg) {
>       if (VM_Version::arm_arch() >= 7) {
>         emit_int32(0xF57FF050 | opt);
> -    } else {
> +    } else if (VM_Version::arm_arch() == 6) {
>         bool preserve_tmp = (reg == noreg);
>         if(preserve_tmp) {
>           reg = Rtemp;
> diff --git a/src/hotspot/os_cpu/linux_arm/orderAccess_linux_arm.hpp
> b/src/hotspot/os_cpu/linux_arm/orderAccess_linux_arm.hpp
> --- a/src/hotspot/os_cpu/linux_arm/orderAccess_linux_arm.hpp
> +++ b/src/hotspot/os_cpu/linux_arm/orderAccess_linux_arm.hpp
> @@ -63,7 +63,7 @@
>        __asm__ volatile (
>        ".word 0xF57FF050 | 0xf" : : : "memory");
>   #endif
> -   } else {
> +   } else if (VM_Version::arm_arch() == 6) {
>        intptr_t zero = 0;
>        __asm__ volatile (
>          "mcr p15, 0, %0, c7, c10, 5"
> @@ -80,7 +80,7 @@
>        __asm__ volatile (
>        ".word 0xF57FF050 | 0xe" : : : "memory");
>   #endif
> -   } else {
> +   } else if (VM_Version::arm_arch() == 6) {
>        intptr_t zero = 0;
>        __asm__ volatile (
>          "mcr p15, 0, %0, c7, c10, 5"
>