Re: [PATCH v3 8/8] ARM: make nr_irqs a constant

[Julien Grall <julien.grall@linaro.org>]

On 01/02/18 14:34, Andre Przywara wrote:
> Hi,

Hi,

> On 01/02/18 13:47, Julien Grall wrote:
>> Hi Andre,
>>
>> On 01/02/18 13:43, Andre Przywara wrote:
>>> On 30/01/18 14:36, Roger Pau Monné wrote:
>>>> On Wed, Jan 24, 2018 at 06:10:58PM +0000, Andre Przywara wrote:
>>>>> On ARM the maximum number of IRQs is a constant, but we share it being
>>>>> a variable to match x86. Since we are not supposed to alter it, let's
>>>>> mark it as "const" to avoid accidental change.
>>>>>
>>>>> Suggested-by: Julien Grall <julien.grall@linaro.org>
>>>>> Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
>>>>> ---
>>>>>    xen/arch/arm/irq.c        | 2 +-
>>>>>    xen/include/asm-arm/irq.h | 2 +-
>>>>>    2 files changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/xen/arch/arm/irq.c b/xen/arch/arm/irq.c
>>>>> index 62103a20e3..d229cb6871 100644
>>>>> --- a/xen/arch/arm/irq.c
>>>>> +++ b/xen/arch/arm/irq.c
>>>>> @@ -27,7 +27,7 @@
>>>>>    #include <asm/gic.h>
>>>>>    #include <asm/vgic.h>
>>>>>    -unsigned int __read_mostly nr_irqs = NR_IRQS;
>>>>> +const unsigned int __read_mostly nr_irqs = NR_IRQS;
>>>>
>>>> Shouldn't you remove the __read_mostly attribute, so the symbol it's
>>>> placed at the .rodata section by the compiler?
>>>
>>> Yes, makes sense, thanks for pointing this out!
>>> const ... __read_mostly sounds somewhat redundant.
>>>
>>> It looks like the compiler does the right thing anyway, as I can't find
>>> nr_irqs in the ELF in any case. Both with and without __read_mostly it
>>> results into the very same binary, actually even without the const.
>>
>> Really? I was expecting const data to be in the section.rodata. Are you
>> suggesting this is landing in the section .data instead?
> 
> Well, for the local case (arm/irq.c) the compiler just does the right
> thing and eliminates the variable completely :
> 
> 00000000000005dc <request_irq>:
>   5dc:   eb1f005f        cmp     x2, xzr
>   5e0:   52807fe5        mov     w5, #0x3ff                // #1023
>   5e4:   7a451002        ccmp    w0, w5, #0x2, ne
>   5e8:   540003e8        b.hi    664 <request_irq+0x88>    // EINVAL
> 
> For common/domain.o it is as you guessed: in .data.read_mostly, with or
> without this (original) patch. So __read_mostly trumps const.
> Removing __read_mostly indeed puts it in .rodata.
> 
> Don't know why the resulting xen/xen.axf don't show any difference, but
> the respective object files do.
xen-syms is an ELF binary.
xen is not an ELF.
xen.axf is not built anymore since Xen 4.9.

That might explain why you are not able to find it.

Cheers,

-- 
Julien Grall

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