Re: [PATCH v1 5/8] cpus-common: ensure auto-assigned cpu_indexes don't clash

[Igor Mammedow <imammedo@redhat.com>]

On Thu, 21 May 2020 18:10:40 +0100
Alex Bennée <alex.bennee@linaro.org> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Thu, 14 May 2020 17:27:53 +0100
> > Alex Bennée <alex.bennee@linaro.org> wrote:
> >  
> >> a
> >> Alex Bennée <alex.bennee@linaro.org> writes:
> >>   
> >> > Basing the cpu_index on the number of currently allocated vCPUs
> >> > fails when vCPUs aren't removed in a LIFO manner. This is
> >> > especially true when we are allocating a cpu_index for each
> >> > guest thread in linux-user where there is no ordering constraint
> >> > on their allocation and de-allocation.
> >> >
> >> > [I've dropped the assert which is there to guard against
> >> > out-of-order removal as this should probably be caught higher up
> >> > the stack. Maybe we could just ifdef CONFIG_SOFTTMU it?]  
> >
> > for machines where we care about cross version migration
> > (arm/virt,s390,x86,spapr), we do manual cpu_index assignment on
> > keep control on its stability So orderining probably shouldn't
> > matter for other softmmu boards, but what I'd watch for is arrays
> > within devices where cpu_index is used as index  
> 
> With the updated version for softmmu you should get the same indexes
> as before from startup. It only gets complicated if CPU hotplug is a
> thing which I think is only the case for machines that also support
> migration?
I'd think so, and those do not (should not) use cpu_get_free_index(), so

Acked-by: Igor Mammedow <imammedo@redhat.com>

> 
> > (ex: would be apic emulation (but its not affected by this patch
> > since x86 control cpu_index assignment))  
> 
> I've just noticed that the gdbstub uses the maximum cpu_index at
> startup to size it's array in CONFIG_USER which is obviously wrong
> but it was wrong before so I guess that's another bug to look into on
> my part :-/
> 
> >
> >  
> >> >
> >> > Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> >> > Cc: Nikolay Igotti <igotti@gmail.com>
> >> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> > Cc: Igor Mammedov <imammedo@redhat.com>
> >> > Cc: Eduardo Habkost <ehabkost@redhat.com>
> >> > ---
> >> >  cpus-common.c | 9 ++++-----
> >> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >> >
> >> > diff --git a/cpus-common.c b/cpus-common.c
> >> > index 55d5df89237..5a7d2f6132b 100644
> >> > --- a/cpus-common.c
> >> > +++ b/cpus-common.c
> >> > @@ -61,13 +61,14 @@ static bool cpu_index_auto_assigned;
> >> >  static int cpu_get_free_index(void)
> >> >  {
> >> >      CPUState *some_cpu;
> >> > -    int cpu_index = 0;
> >> > +    int max_cpu_index = 0;
> >> >  
> >> >      cpu_index_auto_assigned = true;
> >> >      CPU_FOREACH(some_cpu) {
> >> > -        cpu_index++;
> >> > +        max_cpu_index = MAX(some_cpu->cpu_index, max_cpu_index);
> >> >      }
> >> > -    return cpu_index;
> >> > +    max_cpu_index++;
> >> > +    return max_cpu_index;
> >> >  }    
> >> 
> >> OK some ending up with cpu_index = 1 threw off devices that would
> >> do qemu_get_cpu(0) so I've tweaked the algorithm to:
> >> 
> >>   static int cpu_get_free_index(void)
> >>   {
> >>       CPUState *some_cpu;
> >>       int max_cpu_index = 0;
> >> 
> >>       cpu_index_auto_assigned = true;
> >>       CPU_FOREACH(some_cpu) {
> >>           if (some_cpu->cpu_index >= max_cpu_index) {
> >>               max_cpu_index = some_cpu->cpu_index + 1;
> >>           }
> >>       }
> >>       return max_cpu_index;
> >>   }
> >>   
> >> >  
> >> >  void cpu_list_add(CPUState *cpu)
> >> > @@ -90,8 +91,6 @@ void cpu_list_remove(CPUState *cpu)
> >> >          return;
> >> >      }
> >> >  
> >> > -    assert(!(cpu_index_auto_assigned && cpu !=
> >> > QTAILQ_LAST(&cpus))); -
> >> >      QTAILQ_REMOVE_RCU(&cpus, cpu, node);
> >> >      cpu->cpu_index = UNASSIGNED_CPU_INDEX;
> >> >  }    
> >> 
> >>   
> 
>