[PATCH] arm: Hook up SYNC_CORE functionality for sys_membarrier()

[PATCH] arm: Hook up SYNC_CORE functionality for sys_membarrier()

[Will Deacon]

Exception return implies context synchronization, so we can hook up the
SYNC_CORE option to sys_membarrier() simply by selecting the Kconfig option,
just like we've done for arm64 already.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Orion Hodson <oth@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---
 arch/arm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 54eeb8d00bc6..b0ac18547370 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -9,6 +9,7 @@ config ARM
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV
+	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
 	select ARCH_HAS_PHYS_TO_DMA
 	select ARCH_HAS_SET_MEMORY
-- 
2.1.4

[PATCH] arm: Hook up SYNC_CORE functionality for sys_membarrier()

[Mathieu Desnoyers]

----- On Jun 19, 2018, at 8:22 AM, Will Deacon will.deacon at arm.com wrote:

> Exception return implies context synchronization, so we can hook up the
> SYNC_CORE option to sys_membarrier() simply by selecting the Kconfig option,
> just like we've done for arm64 already.
> 
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Orion Hodson <oth@google.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> arch/arm/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 54eeb8d00bc6..b0ac18547370 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -9,6 +9,7 @@ config ARM
> 	select ARCH_HAS_ELF_RANDOMIZE
> 	select ARCH_HAS_FORTIFY_SOURCE
> 	select ARCH_HAS_KCOV
> +	select ARCH_HAS_MEMBARRIER_SYNC_CORE

In addition to this, we added this comment in arch/arm64/kernel/entry.S:

+       /*
+        * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context synchronization
+        * when returning from IPI handler, and when returning to user-space.
+        */

So I would expect a similar comment in arch/arm/kernel/entry-header.S, within
svc_exit and svc_exit_via_fiq:

        /*
         * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on [insn] context synchronization
         * when returning from IPI handler, and when returning to user-space.
         */

Which instruction exactly is responsible for context synchronization on
arm32 ?

Thanks,

Mathieu



> 	select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
> 	select ARCH_HAS_PHYS_TO_DMA
> 	select ARCH_HAS_SET_MEMORY
> --
> 2.1.4

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

[PATCH] arm: Hook up SYNC_CORE functionality for sys_membarrier()

[Will Deacon]

Hi Mathieu,

On Tue, Jun 19, 2018 at 08:50:02AM -0400, Mathieu Desnoyers wrote:
> ----- On Jun 19, 2018, at 8:22 AM, Will Deacon will.deacon at arm.com wrote:
> 
> > Exception return implies context synchronization, so we can hook up the
> > SYNC_CORE option to sys_membarrier() simply by selecting the Kconfig option,
> > just like we've done for arm64 already.
> > 
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Orion Hodson <oth@google.com>
> > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > ---
> > arch/arm/Kconfig | 1 +
> > 1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index 54eeb8d00bc6..b0ac18547370 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -9,6 +9,7 @@ config ARM
> > 	select ARCH_HAS_ELF_RANDOMIZE
> > 	select ARCH_HAS_FORTIFY_SOURCE
> > 	select ARCH_HAS_KCOV
> > +	select ARCH_HAS_MEMBARRIER_SYNC_CORE
> 
> In addition to this, we added this comment in arch/arm64/kernel/entry.S:
> 
> +       /*
> +        * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context synchronization
> +        * when returning from IPI handler, and when returning to user-space.
> +        */
> 
> So I would expect a similar comment in arch/arm/kernel/entry-header.S, within
> svc_exit and svc_exit_via_fiq:
> 
>         /*
>          * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on [insn] context synchronization
>          * when returning from IPI handler, and when returning to user-space.
>          */

Bah, you know I hate that comment ;) I should update arch-support.txt,
though. Diff below.

> Which instruction exactly is responsible for context synchronization on
> arm32 ?

It's the act of doing an exception return, so there are multiple instruction
sequences to do that on 32-bit arm.

Will

--->8

diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
index dbdf62907703..c7858dd1ea8f 100644
--- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
+++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
@@ -5,10 +5,10 @@
 #
 # Architecture requirements
 #
-# * arm64
+# * arm/arm64
 #
-# Rely on eret context synchronization when returning from IPI handler, and
-# when returning to user-space.
+# Rely on implicit context synchronization as a result of exception return
+# when returning from IPI handler, and when returning to user-space.
 #
 # * x86
 #
@@ -31,7 +31,7 @@
     -----------------------
     |       alpha: | TODO |
     |         arc: | TODO |
-    |         arm: | TODO |
+    |         arm: |  ok  |
     |       arm64: |  ok  |
     |         c6x: | TODO |
     |       h8300: | TODO |

[PATCH] arm: Hook up SYNC_CORE functionality for sys_membarrier()

[Mathieu Desnoyers]

----- On Jun 19, 2018, at 8:58 AM, Will Deacon will.deacon at arm.com wrote:

> Hi Mathieu,
> 
> On Tue, Jun 19, 2018 at 08:50:02AM -0400, Mathieu Desnoyers wrote:
>> ----- On Jun 19, 2018, at 8:22 AM, Will Deacon will.deacon at arm.com wrote:
>> 
>> > Exception return implies context synchronization, so we can hook up the
>> > SYNC_CORE option to sys_membarrier() simply by selecting the Kconfig option,
>> > just like we've done for arm64 already.
>> > 
>> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> > Cc: Orion Hodson <oth@google.com>
>> > Signed-off-by: Will Deacon <will.deacon@arm.com>
>> > ---
>> > arch/arm/Kconfig | 1 +
>> > 1 file changed, 1 insertion(+)
>> > 
>> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> > index 54eeb8d00bc6..b0ac18547370 100644
>> > --- a/arch/arm/Kconfig
>> > +++ b/arch/arm/Kconfig
>> > @@ -9,6 +9,7 @@ config ARM
>> > 	select ARCH_HAS_ELF_RANDOMIZE
>> > 	select ARCH_HAS_FORTIFY_SOURCE
>> > 	select ARCH_HAS_KCOV
>> > +	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>> 
>> In addition to this, we added this comment in arch/arm64/kernel/entry.S:
>> 
>> +       /*
>> +        * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on eret context synchronization
>> +        * when returning from IPI handler, and when returning to user-space.
>> +        */
>> 
>> So I would expect a similar comment in arch/arm/kernel/entry-header.S, within
>> svc_exit and svc_exit_via_fiq:
>> 
>>         /*
>>          * ARCH_HAS_MEMBARRIER_SYNC_CORE rely on [insn] context synchronization
>>          * when returning from IPI handler, and when returning to user-space.
>>          */
> 
> Bah, you know I hate that comment ;) I should update arch-support.txt,
> though. Diff below.

It works for me with this update, thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com