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

From: Wei Liu <wei.liu@kernel.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wei Liu <wei.liu@kernel.org>,
	Linux on Hyper-V List <linux-hyperv@vger.kernel.org>,
	virtualization@lists.linux-foundation.org,
	Linux Kernel List <linux-kernel@vger.kernel.org>,
	Michael Kelley <mikelley@microsoft.com>,
	Vineeth Pillai <viremana@linux.microsoft.com>,
	Sunil Muthuswamy <sunilmut@microsoft.com>,
	Nuno Das Neves <nudasnev@microsoft.com>,
	Lillian Grassin-Drake <ligrassi@microsoft.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH RFC v1 10/18] x86/hyperv: implement and use hv_smp_prepare_cpus
Date: Tue, 27 Oct 2020 13:47:57 +0000	[thread overview]
Message-ID: <20201027134757.k2hjvcmq3ezqo4mm@liuwe-devbox-debian-v2> (raw)
In-Reply-To: <87mu1rjnqv.fsf@vitty.brq.redhat.com>

On Tue, Sep 15, 2020 at 01:14:16PM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu@kernel.org> 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@microsoft.com>
> > Signed-off-by: Sunil Muthuswamy <sunilmut@microsoft.com>
> > Co-Developed-by: Lillian Grassin-Drake <ligrassi@microsoft.com>
> > Co-Developed-by: Sunil Muthuswamy <sunilmut@microsoft.com>
> > Signed-off-by: Wei Liu <wei.liu@kernel.org>
> > ---
> > 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.

Wei.

> 
> > +		BUG_ON(ret);
> > +	}
> > +#endif
> > +}
> >  #endif
> >  
> >  static void __init ms_hyperv_init_platform(void)
> > @@ -359,6 +384,8 @@ static void __init ms_hyperv_init_platform(void)
> >  
> >  # ifdef CONFIG_SMP
> >  	smp_ops.smp_prepare_boot_cpu = hv_smp_prepare_boot_cpu;
> > +	if (hv_root_partition)
> > +		smp_ops.smp_prepare_cpus = hv_smp_prepare_cpus;
> >  # endif
> >  
> >  	/*
> 
> -- 
> Vitaly
> 