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

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Message-ID: <20230221055117.GA1934@templeofstupid.com>
Date:   Mon, 20 Feb 2023 21:51:17 -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 08:14:40PM -0800, Krister Johansen wrote:
> 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.

Bah, what I meant to say was emulated, unstable, or otherwise unsuitable
for use as a clocksource.  IOW, even if TSC_MODE_NEVER_EMULATE is
set, it's possible that a user is attempting a migration from a cpu
that's not invariant, and we'd still want to check for that case and
fall back to a PV clocksource, correct?

-K