linux-kernel - Re: [Patch] Skip cpu_calibrate for kernel running under hypervisors.

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Message-Id: <1282063710.4388.11.camel@ank32.eng.vmware.com>
Date:	Tue, 17 Aug 2010 09:48:30 -0700
From:	Alok Kataria <akataria@...are.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	the arch/x86 maintainers <x86@...nel.org>,
	Greg KH <gregkh@...e.de>, "greg@...ah.com" <greg@...ah.com>,
	"ksrinivasan@...ell.com" <ksrinivasan@...ell.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Skip cpu_calibrate for kernel running
	under	hypervisors.

Hi HPA,

On Mon, 2010-08-16 at 23:30 -0700, H. Peter Anvin wrote:
> On 08/16/2010 10:51 PM, Alok Kataria wrote:
> >>
> >> I'm somewhat reluctant to take this one, since it assumes all the
> >> hypervisors act the same.  This seems rather inherently wrong.  In fact,
> >> the whole statement is fishy as heck... instead of being dependent on
> >> AMD and so on, 
> > 
> > The check about being on AMD is something that was already there. 
> > 
> 
> I know it was... and calibrate_cpu() seems to be an AMD-specific
> function, but that's rather crappy.  I'm thinking that perhaps we should
> make it an x86_init function, then the AMD CPU detection can install it
> and the vmware hypervisor detection can uninstall it.

I am planning to add a calibrate_apic function ptr in x86_platform_ops,
for getting the APIC frequency too directly from the hypervisor. If you
want I can add this calibrate_cpu function ptr too or is the patch below
okay for now ? 

Thanks,
Alok

> 
> >> this should either be a function pointer or a CPU
> >> (mis)feature bit.
> > 
> > In any case, I agree that my previous patch did assume all hypervisors
> > to be same, which might be wrong. How about using the
> > X86_FEATURE_TSC_RELIABLE bit for this too ? i.e. Skip cpu_calibrate call
> > if TSC_RELIABLE bit is set. As of now that bit is set for vmware only. 
> > 
> > Something like the below.
> > 
> > Signed-off-by: Alok N Kataria <akataria@...are.com>
> > Cc: H. Peter Anvin <hpa@...or.com>
> > 
> > Index: linux-x86-tree.git/arch/x86/kernel/tsc.c
> > ===================================================================
> > --- linux-x86-tree.git.orig/arch/x86/kernel/tsc.c	2010-08-03 12:21:20.000000000 -0700
> > +++ linux-x86-tree.git/arch/x86/kernel/tsc.c	2010-08-16 21:59:32.000000000 -0700
> > @@ -927,7 +927,8 @@ void __init tsc_init(void)
> >  	}
> >  
> >  	if (cpu_has(&boot_cpu_data, X86_FEATURE_CONSTANT_TSC) &&
> > -			(boot_cpu_data.x86_vendor == X86_VENDOR_AMD))
> > +	    (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) &&
> > +	    !(cpu_has(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE)))
> >  		cpu_khz = calibrate_cpu();
> >  
> >  	printk("Detected %lu.%03lu MHz processor.\n",
> > 
> 
> That seems like a much better approach.
> 
> 	-hpa
> 

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