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

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Date:   Fri, 9 Dec 2022 14:32:15 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Krister Johansen <kjlx@...pleofstupid.com>,
        Juergen Gross <jgross@...e.com>
Cc:     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] x86/xen/time: prefer tsc as clocksource when
 it is invariant


On 12/8/22 11:36 AM, Krister Johansen wrote:
> +	/*
> +	 * As Dom0 is never moved, no penalty on using TSC there.
> +	 *
> +	 * If the guest has invariant tsc, then set xen_clocksource rating
> +	 * below that of the tsc so that the system prefers tsc instead.  This
> +	 * check excludes PV domains, because PV is unable to guarantee that the
> +	 * guest's cpuid call has been intercepted by the hypervisor.
> +	 */
> +	if (xen_initial_domain()) {
>   		xen_clocksource.rating = 275;
> +	} else if ((xen_hvm_domain() || xen_pvh_domain()) &&
> +	    boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> +	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> +	    !check_tsc_unstable()) {
> +		xen_clocksource.rating = 299;
> +	}


What if RDTSC is intercepted?


-boris