linux-kernel - Re: [RFC v2 2/2] x86/kvmclock: Introduce kvm-hostclock clocksource.

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Date:   Thu, 10 Oct 2019 12:55:27 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Suleiman Souhlal <suleiman@...gle.com>, rkrcmar@...hat.com,
        tglx@...utronix.de
Cc:     john.stultz@...aro.org, sboyd@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        ssouhlal@...ebsd.org, tfiga@...omium.org, vkuznets@...hat.com
Subject: Re: [RFC v2 2/2] x86/kvmclock: Introduce kvm-hostclock clocksource.

On 10/10/19 09:30, Suleiman Souhlal wrote:
> +kvm_hostclock_enable(struct clocksource *cs)
> +{
> +	pv_timekeeper_enabled = 1;
> +
> +	old_vclock_mode = kvm_clock.archdata.vclock_mode;
> +	kvm_clock.archdata.vclock_mode = VCLOCK_TSC;
> +	return 0;
> +}
> +
> +static void
> +kvm_hostclock_disable(struct clocksource *cs)
> +{
> +	pv_timekeeper_enabled = 0;
> +	kvm_clock.archdata.vclock_mode = old_vclock_mode;
> +}
> +

Why do you poke at kvm_clock?  Instead you should add

	.archdata               = { .vclock_mode = VCLOCK_TSC },

to the kvm_hostclock declaration.

Please also check that the invariant TSC CPUID bit
CPUID[0x80000007].EDX[8] is set before enabling this feature.

Paolo

> +	pvtk = &pv_timekeeper;
> +	do {
> +		gen = pvtk_read_begin(pvtk);
> +		if (!(pv_timekeeper.flags & PVCLOCK_TIMEKEEPER_ENABLED))
> +			return;
> +
> +		pvclock_copy_into_read_base(pvtk, &tk->tkr_mono,
> +		    &pvtk->tkr_mono);
> +		pvclock_copy_into_read_base(pvtk, &tk->tkr_raw, &pvtk->tkr_raw);
> +
> +		tk->xtime_sec = pvtk->xtime_sec;
> +		tk->ktime_sec = pvtk->ktime_sec;
> +		tk->wall_to_monotonic.tv_sec = pvtk->wall_to_monotonic_sec;
> +		tk->wall_to_monotonic.tv_nsec = pvtk->wall_to_monotonic_nsec;
> +		tk->offs_real = pvtk->offs_real;
> +		tk->offs_boot = pvtk->offs_boot;
> +		tk->offs_tai = pvtk->offs_tai;
> +		tk->raw_sec = pvtk->raw_sec;
> +	} while (pvtk_read_retry(pvtk, gen));
> +}
> +

Should you write an "enabled value" (basically the flags) into pvtk as well?

> 
> +kvm_hostclock_init(void)
> +{
> +	unsigned long pa;
> +
> +	pa = __pa(&pv_timekeeper);
> +	wrmsrl(MSR_KVM_TIMEKEEPER_EN, pa);


As Vitaly said, a new CPUID bit must be defined in
Documentation/virt/kvm/cpuid.txt, and used here.  Also please make bit 0
an enable bit.