linux-kernel - Re: [PATCH linux-next 2/2] x86/xen/time: cleanup xen_tsc_safe

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Message-ID: <20230221041440.GA1934@templeofstupid.com>
Date:   Mon, 20 Feb 2023 20:14:40 -0800
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Juergen Gross <jgross@...e.com>,
        Jan Beulich <jbeulich@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Anthony Liguori <aliguori@...zon.com>,
        David Reaver <me@...idreaver.com>,
        Brendan Gregg <brendan@...el.com>
Subject: Re: [PATCH linux-next 2/2] x86/xen/time: cleanup
 xen_tsc_safe_clocksource

On Mon, Feb 20, 2023 at 11:01:18PM +0100, Thomas Gleixner wrote:
> On Mon, Feb 20 2023 at 09:17, Krister Johansen wrote:
> > @@ -495,8 +496,7 @@ static int __init xen_tsc_safe_clocksource(void)
> >  	/* Leaf 4, sub-leaf 0 (0x40000x03) */
> >  	cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx);
> >  
> > -	/* tsc_mode = no_emulate (2) */
> > -	if (ebx != 2)
> > +	if (ebx != XEN_CPUID_TSC_MODE_NEVER_EMULATE)
> >  		return 0;
> >  
> >  	return 1;
> 
> What about removing more stupidity from that function?
> 
> static bool __init xen_tsc_safe_clocksource(void)
> {
> 	u32 eax, ebx. ecx, edx;
>  
> 	/* Leaf 4, sub-leaf 0 (0x40000x03) */
> 	cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx);
> 
> 	return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE;
> }

I'm all for simplifying.  I'm happy to clean up that return to be more
idiomatic.  I was under the impression, perhaps mistaken, though, that
the X86_FEATURE_CONSTANT_TSC, X86_FEATURE_NONSTOP_TSC, and
check_tsc_unstable() checks were actually serving a purpose: to ensure
that we don't rely on the tsc in environments where it's being emulated
and the OS would be better served by using a PV clock.  Specifically,
kvmclock_init() makes a very similar set of checks that I also thought
were load-bearing.

-K