On 26.02.2021 04:08, Connor Davis wrote:
> On Thu, Feb 25, 2021 at 04:50:02PM +0100, Jan Beulich wrote:
>> On 25.02.2021 16:24, Connor Davis wrote:
>>> Return from cpu_schedule_up when either cpu is 0 or
>>> NR_CPUS == 1. This fixes the following:
>>>
>>> core.c: In function 'cpu_schedule_up':
>>> core.c:2769:19: error: array subscript 1 is above array bounds
>>> of 'struct vcpu *[1]' [-Werror=array-bounds]
>>> 2769 | if ( idle_vcpu[cpu] == NULL )
>>> |
>>>
>>> Signed-off-by: Connor Davis <[email protected]>
>>> ---
>>> xen/common/sched/core.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
>>> index 9745a77eee..f5ec65bf9b 100644
>>> --- a/xen/common/sched/core.c
>>> +++ b/xen/common/sched/core.c
>>> @@ -2763,7 +2763,7 @@ static int cpu_schedule_up(unsigned int cpu)
>>> cpumask_set_cpu(cpu, &sched_res_mask);
>>>
>>> /* Boot CPU is dealt with later in scheduler_init(). */
>>> - if ( cpu == 0 )
>>> + if ( cpu == 0 || NR_CPUS == 1 )
>>> return 0;
>>>
>>> if ( idle_vcpu[cpu] == NULL )
>>
>> I'm not convinced a compiler warning is due here, and in turn
>> I'm not sure we want/need to work around this the way you do.
>
> It seems like a reasonable warning to me, but of course I'm open
> to dealing with it in a different way.
>
>> First question is whether that's just a specific compiler
>> version that's flawed. If it's not just a special case (e.g.
>
> The docker container uses gcc 10.2.0 from
> https://github.com/riscv/riscv-gnu-toolchain
Ah yes, at -O2 I can observe the warning on e.g.
extern int array[N];
int test(unsigned i) {
if(i == N - 1)
return 0;
return array[i];
}
when N=1. No warning appears when N=2 or higher, yet if it is
sensible to emit for N=1 then it would imo be similarly
sensible to emit in other cases. The only difference is that
when N=1, there's no i for which the array access would ever
be valid, while e.g. for N=2 there's exactly one such i.
I've tried an x86 build with NR_CPUS=1, and this hits the case
you found and a 2nd one, where behavior is even more puzzling.
For the case you've found I'd like to suggest as alternative
@@ -2769,6 +2769,12 @@ static int cpu_schedule_up(unsigned int
if ( cpu == 0 )
return 0;
+ /*
+ * Guard in particular also against the compiler suspecting out-of-bounds
+ * array accesses below when NR_CPUS=1.
+ */
+ BUG_ON(cpu >= NR_CPUS);
+
if ( idle_vcpu[cpu] == NULL )
vcpu_create(idle_vcpu[0]->domain, cpu);
else
To fix the x86 build in this regard we'd additionally need
something along the lines of
--- unstable.orig/xen/arch/x86/genapic/x2apic.c
+++ unstable/xen/arch/x86/genapic/x2apic.c
@@ -54,7 +54,17 @@ static void init_apic_ldr_x2apic_cluster
per_cpu(cluster_cpus, this_cpu) = cluster_cpus_spare;
for_each_online_cpu ( cpu )
{
- if (this_cpu == cpu || x2apic_cluster(this_cpu) != x2apic_cluster(cpu))
+ if ( this_cpu == cpu )
+ continue;
+ /*
+ * Guard in particular against the compiler suspecting out-of-bounds
+ * array accesses below when NR_CPUS=1 (oddly enough with gcc 10 it
+ * is the 1st of these alone which actually helps, not the 2nd, nor
+ * are both required together there).
+ */
+ BUG_ON(this_cpu >= NR_CPUS);
+ BUG_ON(cpu >= NR_CPUS);
+ if ( x2apic_cluster(this_cpu) != x2apic_cluster(cpu) )
continue;
per_cpu(cluster_cpus, this_cpu) = per_cpu(cluster_cpus, cpu);
break;
but the comment points out how strangely the compiler behaves here.
Even flipping around the two sides of the != doesn't change its
behavior. It is perhaps relevant to note here that there's no
special casing of smp_processor_id() in the NR_CPUS=1 case, so the
compiler can't infer this_cpu == 0.
Once we've settled on how to change common/sched/core.c I guess
I'll then adjust the x86-specific change accordingly and submit as
a separate fix (or I could of course also bundle both changes then).
Jan
