Re: [PATCH 1/2] asm-generic/hyperv: provide cpumask_to_vpset_noself

[Wei Liu <wei.liu@kernel.org>]

On Thu, Sep 09, 2021 at 07:38:28AM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@kernel.org> writes:
> 
> > This is a new variant which removes `self' cpu from the vpset. It will
> > be used in Hyper-V enlightened IPI code.
> >
> > Signed-off-by: Wei Liu <wei.liu@kernel.org>
> > ---
> > Provide a new variant instead of adding a new parameter because it makes
> > it easier to backport -- we don't need to fix the users of
> > cpumask_to_vpset.
> > ---
> >  include/asm-generic/mshyperv.h | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> > index 9a000ba2bb75..d89690ee95aa 100644
> > --- a/include/asm-generic/mshyperv.h
> > +++ b/include/asm-generic/mshyperv.h
> > @@ -184,10 +184,12 @@ static inline int hv_cpu_number_to_vp_number(int cpu_number)
> >  	return hv_vp_index[cpu_number];
> >  }
> >  
> > -static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> > -				    const struct cpumask *cpus)
> > +static inline int cpumask_to_vpset_ex(struct hv_vpset *vpset,
> 
> I'd suggest to avoid '_ex' suffix as we use it for 'extended hypercalls'
> (e.g. __send_ipi_mask_ex). Assuming nobody needs to call
> cpumask_to_vpset_ex() directly, should we just go with
> __cpumask_to_vpset() instead?

Sure. I'm not too fussed about the name.

I will wait a bit for other people to express their opinions.

> 
> > +				    const struct cpumask *cpus,
> > +				    bool exclude_self)
> >  {
> >  	int cpu, vcpu, vcpu_bank, vcpu_offset, nr_bank = 1;
> > +	int this_cpu = smp_processor_id();
> >  
> >  	/* valid_bank_mask can represent up to 64 banks */
> >  	if (hv_max_vp_index / 64 >= 64)
> > @@ -205,6 +207,8 @@ static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> >  	 * Some banks may end up being empty but this is acceptable.
> >  	 */
> >  	for_each_cpu(cpu, cpus) {
> > +		if (exclude_self && cpu == this_cpu)
> > +			continue;
> >  		vcpu = hv_cpu_number_to_vp_number(cpu);
> >  		if (vcpu == VP_INVAL)
> >  			return -1;
> > @@ -219,6 +223,18 @@ static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> >  	return nr_bank;
> >  }
> >  
> > +static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> > +				    const struct cpumask *cpus)
> > +{
> > +	return cpumask_to_vpset_ex(vpset, cpus, false);
> > +}
> > +
> > +static inline int cpumask_to_vpset_noself(struct hv_vpset *vpset,
> > +				    const struct cpumask *cpus)
> > +{
> > +	return cpumask_to_vpset_ex(vpset, cpus, true);
> 
> 
> We need to make sure this is called with preemption disabled. We
> could've just swapped smp_processor_id() for get_cpu() in
> cpumask_to_vpset_ex() but this is hardly a solution: we can get
> preempted right after put_cpu() so it's really the caller of this
> function which needs to be protected.
> 
> TL;DR: I suggest we add 'WARN_ON_ONCE(preemptible());' or something like
> this here to catch wrong usage.

Good suggestion. I can add this check to the noself variant. Or if
people prefer, this check can also be moved into the leaf helper.

Wei.

> 
> > +}
> > +
> >  void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die);
> >  bool hv_is_hyperv_initialized(void);
> >  bool hv_is_hibernation_supported(void);
> 
> -- 
> Vitaly
>