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

[Andre Przywara <andre.przywara@xxxxxxxxxx>]

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@xxxxxxxxxx>
>>>> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
>>>> ---
>>>>   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.

Cheers,
Andre.

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