linux-kernel - Re: [PATCH 2/2] KVM: x86: use raw clock values consistently

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Message-ID: <ed5b7a36-b9a0-5436-c704-58c65966b7c2@redhat.com>
Date:   Thu, 23 Jan 2020 14:54:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     mtosatti@...hat.com, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: use raw clock values consistently

On 23/01/20 14:43, Vitaly Kuznetsov wrote:
>> +
>> +static s64 get_kvmclock_base_ns(void)
>> +{
>> +	/* Count up from boot time, but with the frequency of the raw clock.  */
>> +	return ktime_to_ns(ktime_add(ktime_get_raw(), pvclock_gtod_data.offs_boot));
>> +}
>> +#else
>> +static s64 get_kvmclock_base_ns(void)
>> +{
>> +	/* Master clock not used, so we can just use CLOCK_BOOTTIME.  */
>> +	return ktime_get_boottime_ns();
>> +}
>>  #endif
> But we could've still used the RAW+offs_boot version, right? And this is
> just to basically preserve the existing behavior on !x86.

Yes, there's no reason to restrict the pvclock_gtod notifier to x86_64.
 But this is stable material so I kept it easy.

>>
>> -	getboottime64(&boot);
>> +	wall_nsec = ktime_get_real_ns() - get_kvmclock_ns(kvm);
> 
> There are not that many hosts with more than 50 years uptime and likely
> none running Linux with live kernel patching support so I bet noone will
> ever see this overflowing, however, as wall_nsec is u64 and we're
> dealing with kvmclock here I'd suggest to add a WARN_ON().

You're off by a factor of 10, 2^64 nanoseconds are about 584 years
(584*365*10^9*86400). :)

Paolo