Re: [PATCH RFC v1 10/18] x86/hyperv: implement and use hv_smp_prepare_cpus

[Wei Liu]

On Tue, Oct 27, 2020 at 01:47:57PM +0000, Wei Liu wrote:
> On Tue, Sep 15, 2020 at 01:14:16PM +0200, Vitaly Kuznetsov wrote:
> > Wei Liu <wei.liu@xxxxxxxxxx> writes:
> > 
> > > Microsoft Hypervisor requires the root partition to make a few
> > > hypercalls to setup application processors before they can be used.
> > >
> > > Signed-off-by: Lillian Grassin-Drake <ligrassi@xxxxxxxxxxxxx>
> > > Signed-off-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> > > Co-Developed-by: Lillian Grassin-Drake <ligrassi@xxxxxxxxxxxxx>
> > > Co-Developed-by: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> > > Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> > > ---
> > > CPU hotplug and unplug is not yet supported in this setup, so those
> > > paths remain untouched.
> > > ---
> > >  arch/x86/kernel/cpu/mshyperv.c | 27 +++++++++++++++++++++++++++
> > >  1 file changed, 27 insertions(+)
> > >
> > > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > > index 1bf57d310f78..7522cae02759 100644
> > > --- a/arch/x86/kernel/cpu/mshyperv.c
> > > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > > @@ -203,6 +203,31 @@ static void __init hv_smp_prepare_boot_cpu(void)
> > >  	hv_init_spinlocks();
> > >  #endif
> > >  }
> > > +
> > > +static void __init hv_smp_prepare_cpus(unsigned int max_cpus)
> > > +{
> > > +#if defined(CONFIG_X86_64)
> > 
> > I think it makes little sense to try to make Linux work as Hyper-V root
> > partition when !CONFIG_X86_64. If we still care about Hyper-V enablement
> > for !CONFIG_X86_64 we can probably introduce something like
> > CONFIG_HYPERV_ROOT and enable it automatically, e.g.
> > 
> > config HYPERV_ROOT
> >         def_bool HYPERV && X86_64
> > 
> > and use it instead.
> > 
> 
> We have a patch for such a config option in the /dev/mshv patch set. But
> that's not yet included here so I will keep this as-is.
> 
> > > +	int i;
> > > +	int vp_index = 1;
> > > +	int ret;
> > > +
> > > +	native_smp_prepare_cpus(max_cpus);
> > > +
> > > +	for_each_present_cpu(i) {
> > > +		if (i == 0)
> > > +			continue;
> > > +		ret = hv_call_add_logical_proc(numa_cpu_node(i), i, cpu_physical_id(i));
> > > +		BUG_ON(ret);
> > > +	}
> > > +
> > > +	for_each_present_cpu(i) {
> > > +		if (i == 0)
> > > +			continue;
> > > +		ret = hv_call_create_vp(numa_cpu_node(i), hv_current_partition_id, vp_index++, i);
> > 
> > So vp_index variable is needed here to make sure there are no gaps? (or
> > we could've just used 'i')?
> 
> Not sure. I didn't write the original code in this function. The last
> argument (i) is the logical processor index.
> 
> I don't see a reason why vp_index and lp_index can't be the same. I will
> try dropping vp_index. If that works then great; if not, I will keep the
> code as-is.
> 
> Sunil, if you have more insight, please chime in.
> 

A quick test shows that replacing vp_index with i works just fine.

Wei.