Re: [PATCH v2] xen/arm: Set correct per-cpu cpu_core_mask

[Henry Wang <xin.wang2@xxxxxxx>]

Hi Julien,

On 3/15/2024 6:58 PM, Julien Grall wrote:
> On 14/03/2024 14:22, Henry Wang wrote:
> > Hi Julien,
> Hi,
> > > >   /* representing HT siblings of each logical CPU */
> > > >   DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_mask);
> > > >   /* representing HT and core siblings of each logical CPU */
> > > > @@ -89,6 +88,10 @@ static int setup_cpu_sibling_map(int cpu)
> > > >       cpumask_set_cpu(cpu, per_cpu(cpu_sibling_mask, cpu));
> > > >       cpumask_set_cpu(cpu, per_cpu(cpu_core_mask, cpu));
> > > >   +    /* Currently we assume there is no multithread. */
> > > I am not very familiar with the scheduling in Xen. Do you know 
> > > what's the consequence of not properly supporting MT? One thing I 
> > > can think of is security (I know there were plenty of security 
> > > issues with SMT).
> > Unfortunately me neither, so adding George to this thread as I think 
> > he can bring us some insights on this topic from the scheduler 
> > perspective.
> +Juergen as he worked on co-scheduling.
Thanks.

> > > Depending on the answer, I would consider to print a warning and 
> > > maybe add it in SUPPORT.MD in a separate patch.
> > To be honest, as discussed in v1. I think I am quite tempted to add 
> > an ASSERT(system_cpuinfo.mpidr.mt == 0) to make sure we catch the 
> > multithread support stuff earlier. 
> ASSERT(...) is not the right solution. You may have user using a Xen 
> shipped by distros where this would be a NOP.
> We could try to hide MT completely from the scheduler. If that's too 
> difficult, then we could add use warning_add() to throw a warning (see 
> how we dealt with opt_hmp_unsafe).
Ok. Let's first see what George and Juergen will say, if MT cannot be 
hidden from scheduler for Arm, maybe we can add something like below:
if ( system_cpuinfo.mpidr.mt == 1 )
     warning_add("WARNING: MULTITHREADING HAS BEEN DETECTED ON THE 
PROCESSOR.\n"
                              "It might impact the security and 
stability of the system.\n");
Kind regards,
Henry