Re: [webkit-dev] Data Memory Barrier ARMv6 question

[JF Bastien]

> On Jul 5, 2017, at 19:19, Caio Lima  wrote:
> 
> 2017-07-05 12:41 GMT-03:00 JF Bastien :
>> On Linux you can do the following:
>> 
>> ((void(*)())0x0fa0)();
>> 
>> 
>> That address contains a helper which does the “right” barrier, including if
>> you’re not on an SMP system it’ll do nothing.
>> 
>> Details:
>> https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
>> That file also lists other Linux helpers.
>> 
>> I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier
>> instruction ins’t supported by all ARMv6 CPUs.
> 
> Hi JF. Do you mind point me where the mcr isn't supported by all ARMv6
> CPU? I've found in ARMv6-M manual
> (http://infocenter.arm.com/help/topic/com.arm.doc.ddi0419d/DDI0419D_armv6m_arm.pdf)
> that it doesn't support coprocessor operations, but this architecture
> is used by microcontroller chips.

I unfortunately don’t have a reference for this, but yes IIUC it’s either UP 
systems or non-A profile CPUs.

> Regards,
> Caio.
> 
>> For ARMv7 and later, DMB ish is the right thing.
>> 
>> 
>> On Jul 3, 2017, at 17:19, Caio Lima  wrote:
>> 
>> Hi all.
>> 
>> I'm working in this patch
>> (https://bugs.webkit.org/show_bug.cgi?id=172767) and Mark Lam raised
>> some questions about the data memory barrier (DMB for short) in ARMv6
>> using "mcr 15 ...". The point is that we are having divergences in ARM
>> official reference manual about the semantics of this instruction. We
>> have it discussed in the bug above and I would like to know if there
>> is somebody with stronger ARM background that could help us there and
>> then approve the patch to be committed.
>> 
>> I thanks in advance and best regards,
>> Caio Lima.
>> ___
>> webkit-dev mailing list
>> webkit-dev@lists.webkit.org
>> https://lists.webkit.org/mailman/listinfo/webkit-dev
>> 
>> 

___
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev

Re: [webkit-dev] Data Memory Barrier ARMv6 question

[Caio Lima]

2017-07-05 12:41 GMT-03:00 JF Bastien :
> On Linux you can do the following:
>
> ((void(*)())0x0fa0)();
>
>
> That address contains a helper which does the “right” barrier, including if
> you’re not on an SMP system it’ll do nothing.
>
> Details:
> https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
> That file also lists other Linux helpers.
>
> I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier
> instruction ins’t supported by all ARMv6 CPUs.

Hi JF. Do you mind point me where the mcr isn't supported by all ARMv6
CPU? I've found in ARMv6-M manual
(http://infocenter.arm.com/help/topic/com.arm.doc.ddi0419d/DDI0419D_armv6m_arm.pdf)
that it doesn't support coprocessor operations, but this architecture
is used by microcontroller chips.

Regards,
Caio.

> For ARMv7 and later, DMB ish is the right thing.
>
>
> On Jul 3, 2017, at 17:19, Caio Lima  wrote:
>
> Hi all.
>
> I'm working in this patch
> (https://bugs.webkit.org/show_bug.cgi?id=172767) and Mark Lam raised
> some questions about the data memory barrier (DMB for short) in ARMv6
> using "mcr 15 ...". The point is that we are having divergences in ARM
> official reference manual about the semantics of this instruction. We
> have it discussed in the bug above and I would like to know if there
> is somebody with stronger ARM background that could help us there and
> then approve the patch to be committed.
>
> I thanks in advance and best regards,
> Caio Lima.
> ___
> webkit-dev mailing list
> webkit-dev@lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev
>
>
___
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev

Re: [webkit-dev] Data Memory Barrier ARMv6 question

[JF Bastien]

Alternatively, the JIT can read the content of these addresses and write them 
back as-is 

> On Jul 5, 2017, at 09:01, Filip Pizlo  wrote:
> 
> We should not use those helpers, especially in the JIT. It does not make 
> sense for the JIT to emit calls to system functions when the user is 
> expecting it to emit an instruction. If we cannot perfectly select the right 
> barrier on a particular CPU, we should disable concurrency on that CPU. 
> 
> -Filip
> 
> On Jul 5, 2017, at 8:41 AM, JF Bastien  > wrote:
> 
>> On Linux you can do the following:
>> ((void(*)())0x0fa0)();
>> 
>> That address contains a helper which does the “right” barrier, including if 
>> you’re not on an SMP system it’ll do nothing.
>> 
>> Details: 
>> https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt 
>> 
>> That file also lists other Linux helpers.
>> 
>> I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier 
>> instruction ins’t supported by all ARMv6 CPUs.
>> 
>> For ARMv7 and later, DMB ish is the right thing.
>> 
>> 
>>> On Jul 3, 2017, at 17:19, Caio Lima >> > wrote:
>>> 
>>> Hi all.
>>> 
>>> I'm working in this patch
>>> (https://bugs.webkit.org/show_bug.cgi?id=172767 
>>> ) and Mark Lam raised
>>> some questions about the data memory barrier (DMB for short) in ARMv6
>>> using "mcr 15 ...". The point is that we are having divergences in ARM
>>> official reference manual about the semantics of this instruction. We
>>> have it discussed in the bug above and I would like to know if there
>>> is somebody with stronger ARM background that could help us there and
>>> then approve the patch to be committed.
>>> 
>>> I thanks in advance and best regards,
>>> Caio Lima.
>>> ___
>>> webkit-dev mailing list
>>> webkit-dev@lists.webkit.org 
>>> https://lists.webkit.org/mailman/listinfo/webkit-dev 
>>> 
>> 
>> ___
>> webkit-dev mailing list
>> webkit-dev@lists.webkit.org 
>> https://lists.webkit.org/mailman/listinfo/webkit-dev 
>> 

___
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev

Re: [webkit-dev] Data Memory Barrier ARMv6 question

[Filip Pizlo]

We should not use those helpers, especially in the JIT. It does not make sense 
for the JIT to emit calls to system functions when the user is expecting it to 
emit an instruction. If we cannot perfectly select the right barrier on a 
particular CPU, we should disable concurrency on that CPU. 

-Filip

> On Jul 5, 2017, at 8:41 AM, JF Bastien  wrote:
> 
> On Linux you can do the following:
> ((void(*)())0x0fa0)();
> 
> That address contains a helper which does the “right” barrier, including if 
> you’re not on an SMP system it’ll do nothing.
> 
> Details: https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt
> That file also lists other Linux helpers.
> 
> I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier 
> instruction ins’t supported by all ARMv6 CPUs.
> 
> For ARMv7 and later, DMB ish is the right thing.
> 
> 
>> On Jul 3, 2017, at 17:19, Caio Lima  wrote:
>> 
>> Hi all.
>> 
>> I'm working in this patch
>> (https://bugs.webkit.org/show_bug.cgi?id=172767) and Mark Lam raised
>> some questions about the data memory barrier (DMB for short) in ARMv6
>> using "mcr 15 ...". The point is that we are having divergences in ARM
>> official reference manual about the semantics of this instruction. We
>> have it discussed in the bug above and I would like to know if there
>> is somebody with stronger ARM background that could help us there and
>> then approve the patch to be committed.
>> 
>> I thanks in advance and best regards,
>> Caio Lima.
>> ___
>> webkit-dev mailing list
>> webkit-dev@lists.webkit.org
>> https://lists.webkit.org/mailman/listinfo/webkit-dev
> 
> ___
> webkit-dev mailing list
> webkit-dev@lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev
___
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev

Re: [webkit-dev] Data Memory Barrier ARMv6 question

[JF Bastien]

On Linux you can do the following:
((void(*)())0x0fa0)();

That address contains a helper which does the “right” barrier, including if 
you’re not on an SMP system it’ll do nothing.

Details: https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt 

That file also lists other Linux helpers.

I think for ARMv6 it makes sense to use these helpers. AFAIK the mcr barrier 
instruction ins’t supported by all ARMv6 CPUs.

For ARMv7 and later, DMB ish is the right thing.


> On Jul 3, 2017, at 17:19, Caio Lima  wrote:
> 
> Hi all.
> 
> I'm working in this patch
> (https://bugs.webkit.org/show_bug.cgi?id=172767) and Mark Lam raised
> some questions about the data memory barrier (DMB for short) in ARMv6
> using "mcr 15 ...". The point is that we are having divergences in ARM
> official reference manual about the semantics of this instruction. We
> have it discussed in the bug above and I would like to know if there
> is somebody with stronger ARM background that could help us there and
> then approve the patch to be committed.
> 
> I thanks in advance and best regards,
> Caio Lima.
> ___
> webkit-dev mailing list
> webkit-dev@lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev

___
webkit-dev mailing list
webkit-dev@lists.webkit.org
https://lists.webkit.org/mailman/listinfo/webkit-dev