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

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Message-ID: <871qmjpegh.ffs@tglx>
Date:   Tue, 21 Feb 2023 10:14:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Krister Johansen <kjlx@...pleofstupid.com>
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 21:51, Krister Johansen wrote:
> On Mon, Feb 20, 2023 at 08:14:40PM -0800, Krister Johansen wrote:
>> > 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?

Sure. But a life migration from a NEVER_EMULATE to a non-invariant host
is a patently bad idea and has nothing to do with the __init function,
which is gone at that point already.

What I wanted to say:

static bool __init xen_tsc_safe_clocksource(void)
{
        ......        

	/* 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 didn't have the full context and was just looking at the condition.
Now I checked the full context and I think that except for the

	if (check_tsc_unstable())

check everything else can go away unless you do not trust the hypervisor
that it only sets the NEVER_EMULATE bit when CONSTANT and NONSTOP are
set as well. But yeah, you might prefer to be paranoid. It's virt after
all.

Thanks,

        tglx