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

[Henry Wang <xin.wang2@xxxxxxx>]

Hi Michal,

On 2/26/2024 6:43 PM, Michal Orzel wrote:
> On 26/02/2024 10:54, Wang, Henry wrote:
> > > >   xen/arch/arm/smpboot.c | 6 +++++-
> > > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > +    /* PE not implemented using a multithreading type approach. */
> > > > +    if ( system_cpuinfo.mpidr.mt == 0 )
> > > Do we need this check? It mt was true, cpu_sibling_mask would be incorrect
> > > anyway (it would still be 1).
> > I added this check for playing safe, because I only want to do the correct thing
> > in this patch and avoid make things worse for MT case. With this patch, non-MT
> > case can be improved and the MT case is remain unchanged.
> >
> > But I agree with you, and I would be more than happy if I can run a MT setup and
> > finish the "else" part with this patch or follow-ups. Do you know maybe qemu can
> > allow me to emulate a MT setup so that I can fix it properly in v2? Thanks!
> A65 is the only Arm CPU with SMT and I'm not aware of Qemu being able to 
> emulate it.
> AFAICT, in Xen on Arm we assume no SMT, hence my question about your check. 
> With or without it,
> some parts would still be incorrect (like cpu_sibling_mask), so what's the 
> point in having a partial check.
> I would keep your solution without the check. Others may have a different 
> opinion though.
You made a point. So I think we can wait a bit more for others to share 
their thoughts and I will change accordingly after.
Just taking a note from the chat that we just had - maybe remove the if 
check and add an ASSERT() of the non-MT would also be a good idea (not 
sure if this change will break some existing HW setups though).
Kind regards,
Henry

> ~Michal