Re: [PATCH v1] x86: hyperv: report value of misc_features

From: Wei Liu <wei.liu@kernel.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Olaf Hering <olaf@aepfle.de>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Wei Liu <wei.liu@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-hyperv@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Michael Kelley <mikelley@microsoft.com>
Subject: Re: [PATCH v1] x86: hyperv: report value of misc_features
Date: Thu, 9 Apr 2020 17:26:39 +0100	[thread overview]
Message-ID: <20200409162639.x7vlj2z7h7wt5d4q@debian> (raw)
In-Reply-To: <874ktu2csk.fsf@vitty.brq.redhat.com>

On Wed, Apr 08, 2020 at 05:54:35PM +0200, Vitaly Kuznetsov wrote:
> Olaf Hering <olaf@aepfle.de> writes:
> 
> > A few kernel features depend on ms_hyperv.misc_features, but unlike its
> > siblings ->features and ->hints, the value was never reported during boot.
> >
> > Signed-off-by: Olaf Hering <olaf@aepfle.de>
> > ---
> >  arch/x86/kernel/cpu/mshyperv.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> > index caa032ce3fe3..53706fb56433 100644
> > --- a/arch/x86/kernel/cpu/mshyperv.c
> > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > @@ -227,8 +227,8 @@ static void __init ms_hyperv_init_platform(void)
> >  	ms_hyperv.misc_features = cpuid_edx(HYPERV_CPUID_FEATURES);
> >  	ms_hyperv.hints    = cpuid_eax(HYPERV_CPUID_ENLIGHTMENT_INFO);
> >  
> > -	pr_info("Hyper-V: features 0x%x, hints 0x%x\n",
> > -		ms_hyperv.features, ms_hyperv.hints);
> > +	pr_info("Hyper-V: features 0x%x, hints 0x%x, misc 0x%x\n",
> > +		ms_hyperv.features, ms_hyperv.hints, ms_hyperv.misc_features);
> 
> Several spare thoughts:
> 
> I'm always struggling to remember what is what here as these names don't
> correspond to TLFS, could we maybe come up with better naming, e.g.
> 
> "Features.EAX", "Features.EDX", "Recommendations.EAX",...
> 
> On the other hand, there is more, e.g. nested virtualization
> features. How is it useful for all users to see this in dmesg every
> time? If we need it for development purposes, we can always run 'cpuid'
> so maybe let's drop this altogether or print only with pr_debug? 
> 
> (Honestly: no strong opinion here, deferring to Microsoft folks).

I think making them closer to TLFS is a good idea.

Leaving them always printed is not a big concern to me. Sometimes you
don't get to run `cpuid` if the kernel crashes early.

Olaf's patch is fine as-is. I'm happy to take this patch (if not already
taken by x86 maintainers) given it makes at least one developer's life
easier.

Wei.

> 
> >  
> >  	ms_hyperv.max_vp_index = cpuid_eax(HYPERV_CPUID_IMPLEMENT_LIMITS);
> >  	ms_hyperv.max_lp_index = cpuid_ebx(HYPERV_CPUID_IMPLEMENT_LIMITS);
> >
> 
> -- 
> Vitaly
> 