linux-kernel - Re: [PATCH linux-next v2] x86/xen/time: prefer tsc as clocksource when it is invariant

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Date:   Mon, 12 Dec 2022 14:09:59 -0800
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Krister Johansen <kjlx@...pleofstupid.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        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 v2] x86/xen/time: prefer tsc as clocksource
 when it is invariant

On Mon, Dec 12, 2022 at 01:48:24PM -0500, Boris Ostrovsky wrote:
> 
> On 12/12/22 11:05 AM, Krister Johansen wrote:
> > 
> > diff --git a/arch/x86/include/asm/xen/cpuid.h b/arch/x86/include/asm/xen/cpuid.h
> > index 6daa9b0c8d11..d9d7432481e9 100644
> > --- a/arch/x86/include/asm/xen/cpuid.h
> > +++ b/arch/x86/include/asm/xen/cpuid.h
> > @@ -88,6 +88,12 @@
> >    *             EDX: shift amount for tsc->ns conversion
> >    * Sub-leaf 2: EAX: host tsc frequency in kHz
> >    */
> > +#define XEN_CPUID_TSC_EMULATED       (1u << 0)
> > +#define XEN_CPUID_HOST_TSC_RELIABLE  (1u << 1)
> > +#define XEN_CPUID_RDTSCP_INSTR_AVAIL (1u << 2)
> > +#define XEN_CPUID_TSC_MODE_DEFAULT   (0)
> > +#define XEN_CPUID_TSC_MODE_EMULATE   (1u)
> > +#define XEN_CPUID_TSC_MODE_NOEMULATE (2u)
> 
> This file is a copy of Xen public interface so this change should go to Xen first.

Ok, should I split this into a separate patch on the linux side too?

> > +static int __init xen_tsc_safe_clocksource(void)
> > +{
> > +	u32 eax, ebx, ecx, edx;
> > +
> > +	if (!(xen_hvm_domain() || xen_pvh_domain()))
> > +		return 0;
> > +
> > +	if (!(boot_cpu_has(X86_FEATURE_CONSTANT_TSC)))
> > +		return 0;
> > +
> > +	if (!(boot_cpu_has(X86_FEATURE_NONSTOP_TSC)))
> > +		return 0;
> > +
> > +	if (check_tsc_unstable())
> > +		return 0;
> > +
> > +	cpuid(xen_cpuid_base() + 3, &eax, &ebx, &ecx, &edx);
> > +
> > +	if (eax & XEN_CPUID_TSC_EMULATED)
> > +		return 0;
> > +
> > +	if (ebx != XEN_CPUID_TSC_MODE_NOEMULATE)
> > +		return 0;
> 
> Why is the last test needed?

I was under the impression that if the mode was 0 (default) it would be
possible for the tsc to become emulated in the future, perhaps after a
migration.  The presence of the tsc_mode noemulate meant that we could
count on the falseneess of the XEN_CPUID_TSC_EMULATED check remaining
constant.

-K