linux-kernel - Re: [PATCH 04/22] ARM: LPAE: support 64-bit virt/phys patching

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Date:	Sun, 5 Aug 2012 10:21:17 -0400
From:	Cyril Chemparathy <cyril@...com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <arnd@...db.de>,
	<catalin.marinas@....com>, <linux@....linux.org.uk>,
	<will.deacon@....com>
Subject: Re: [PATCH 04/22] ARM: LPAE: support 64-bit virt/phys patching

Hi Nicolas,

On 8/4/2012 2:49 AM, Nicolas Pitre wrote:
> On Tue, 31 Jul 2012, Cyril Chemparathy wrote:
>
>> This patch adds support for 64-bit physical addresses in virt_to_phys
>> patching.  This does not do real 64-bit add/sub, but instead patches in the
>> upper 32-bits of the phys_offset directly into the output of virt_to_phys.
>
> You should explain _why_ you do not a real aadd/sub.  I did deduce it
> but that might not be obvious to everyone.  Also this subtlety should be
> commented in the code as well.
>

We could not do an ADDS + ADC here because the carry is not guaranteed 
to be retained and passed into the ADC.  This is because the compiler is 
free to insert all kinds of stuff between the two non-volatile asm blocks.

Is there another subtlety here that I have missed out on entirely?

>> In addition to adding 64-bit support, this patch also adds a set_phys_offset()
>> helper that is needed on architectures that need to modify PHYS_OFFSET during
>> initialization.
>>
>> Signed-off-by: Cyril Chemparathy <cyril@...com>
>> ---
>>   arch/arm/include/asm/memory.h |   22 +++++++++++++++-------
>>   arch/arm/kernel/head.S        |    6 ++++++
>>   arch/arm/kernel/setup.c       |   14 ++++++++++++++
>>   3 files changed, 35 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
>> index 4a0108f..110495c 100644
>> --- a/arch/arm/include/asm/memory.h
>> +++ b/arch/arm/include/asm/memory.h
>> @@ -153,23 +153,31 @@
>>   #ifdef CONFIG_ARM_PATCH_PHYS_VIRT
>>
>>   extern unsigned long __pv_phys_offset;
>> -#define PHYS_OFFSET __pv_phys_offset
>> -
>> +extern unsigned long __pv_phys_offset_high;
>
> As mentioned previously, this is just too ugly.  Please make
> __pv_phys_offset into a phys_addr_t instead and mask the low/high parts
> as needed in __virt_to_phys().
>

Maybe u64 instead of phys_addr_t to keep the sizing non-variable?

>>   extern unsigned long __pv_offset;
>>
>> +extern void set_phys_offset(phys_addr_t po);
>> +
>> +#define PHYS_OFFSET	__virt_to_phys(PAGE_OFFSET)
>> +
>>   static inline phys_addr_t __virt_to_phys(unsigned long x)
>>   {
>> -	unsigned long t;
>> -	early_patch_imm8(x, t, "add", __pv_offset);
>> -	return t;
>> +	unsigned long tlo, thi = 0;
>> +
>> +	early_patch_imm8(x, tlo, "add", __pv_offset);
>> +	if (sizeof(phys_addr_t) > 4)
>> +		early_patch_imm8(0, thi, "add", __pv_phys_offset_high);
>
> Given the high part is always the same, isn't there a better way than an
> add with 0 that could be done here?  The add will force a load of 0 in a
> register needlessly just to add a constant value to it.  Your new
> patching framework ought to be able to patch a mov (or a mvn)
> instruction directly.
>

True.  I'll try and figure out a better way of doing this.

>
> Nicolas
>

Once again, thanks for the excellent feedback.

-- 
Thanks
- Cyril
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