linux-kernel - Re: [PATCH 2/2] timers: retpoline mitigation for time funcs

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Date:   Mon, 11 Apr 2022 13:07:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pete Swain <swine@...gle.com>
Cc:     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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Johan Hovold <johan@...nel.org>,
        Feng Tang <feng.tang@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Pete Swain <swine@...gle.com>
Subject: Re: [PATCH 2/2] timers: retpoline mitigation for time funcs

On Sun, Apr 10 2022 at 14:14, Thomas Gleixner wrote:
> On Fri, Feb 18 2022 at 14:18, Pete Swain wrote:
>> @@ -245,7 +245,8 @@ static int clockevents_program_min_delta(struct clock_event_device *dev)
>>  
>>  		dev->retries++;
>>  		clc = ((unsigned long long) delta * dev->mult) >> dev->shift;
>> -		if (dev->set_next_event((unsigned long) clc, dev) == 0)
>> +		if (INDIRECT_CALL_1(dev->set_next_event, lapic_next_deadline,
>> +				  (unsigned long) clc, dev) == 0)
>
> No. We are not sprinkling x86'isms into generic code.

Even if we would do that, then this is completely useless.

The hotpath function is clockevents_program_event() and not
clockevents_program_min_delta().

The latter is invoked from the former if the to be programmed event is
in the past _and_ the force argument is set, i.e. in 0.1% of the
invocations of clockevents_program_event(), which itself has an indirect
call to dev->set_next_event().

If your profiles show clockevents_program_min_delta() as dominant, then
there is something massively wrong with your kernel.

Thanks,

        tglx