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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Feb 01, 2018 at 01:43:09PM +0000, Andre Przywara wrote:
> Hi,
> 
> 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.
> But I will include the change anyway.

Hm, that's kind of weird. nr_irqs seems to be used in ARM code. How
did you assert that the symbol is not there?

This is what I do on x86:

# nm xen/xen-syms | grep ' nr_irqs$'
ffff82d0804324f0 D nr_irqs

Which matches what I would expect from the x86 build.

Roger.

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