Re: [Xen-devel] [PATCH 4/7] xen/arm: gic: Use the correct CPU ID

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-09-10 at 16:42 +0100, Julien Grall wrote:
> On 09/09/2013 02:28 PM, Ian Campbell wrote:
> > On Fri, 2013-08-30 at 14:30 +0100, Julien Grall wrote:
> >> The GIC mapping of CPU interfaces does not necessarily match the logical
> >> CPU numbering.
> >>
> >> When Xen wants to send an SGI to specific CPU, it needs to use the GIC CPU 
> >> ID.
> >> It can be retrieved from ITARGETSR0, in fact when this field is read, the 
> >> GIC
> >> will return a value that corresponds only to the processor reading the 
> >> register.
> >> So Xen can use the PPI 0 to initialize the mapping.
> >>
> >> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
> >> ---
> >>  xen/arch/arm/gic.c |   35 ++++++++++++++++++++++++++++-------
> >>  1 file changed, 28 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
> >> index cadc258..4f3a8a5 100644
> >> --- a/xen/arch/arm/gic.c
> >> +++ b/xen/arch/arm/gic.c
> >> @@ -57,6 +57,27 @@ static DEFINE_PER_CPU(uint64_t, lr_mask);
> >>  
> >>  static unsigned nr_lrs;
> >>  
> >> +/* The GIC mapping of CPU interfaces does not necessarily match the
> >> + * logical CPU numbering. Let's use mapping as returned by the GIC
> >> + * itself
> >> + */
> >> +#define NR_GIC_CPU_IF 8
> >> +static u8 gic_cpu_map[NR_GIC_CPU_IF] __read_mostly = {0xff};
> > 
> > Isn't this mapping from logical cpus ids to gic cpus ids? IOW the size
> > of this array is the wrong way around?
> 
> This array maps a logical cpu id (the index) to a gic cpus id. The size
> if correct, because Xen will allocate logical cpu id from 0 to 7.

Oh right, yes.

I think NR_CPUS (which should be 8 given the other limitations) is the
right size for this array.

> 
> > Is what you want here is a per-cpu variable?
> 
> Does per-cpu variable allow any cpu to retrieve data of another cpu? I
> didn't find a such function.

per_cpu(variable, cpu) will do it.

> 
> >> +
> >> +static unsigned int gic_cpu_mask(const cpumask_t *cpumask)
> >> +{
> >> +    unsigned int cpu;
> >> +    unsigned int mask = 0;
> >> +
> >> +    for_each_cpu(cpu, cpumask)
> >> +    {
> >> +        ASSERT(cpu < NR_GIC_CPU_IF);
> >> +        mask |= gic_cpu_map[cpu];
> > 
> > Further to above, can't cpu here be e.g. 0x100 for an a7 on a big.ITTLE
> > system?
> 
> 0x100 is the hardware CPU id. Xen will allocate a logical cpu id from 0.

Right.



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