linux-kernel - Re: [RFC PATCH 0/2] kvmclock: fix ABI breakage from PVCLOCK_COUNTS_FROM

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Message-ID: <20150922004019.GA25115@amt.cnet>
Date:	Mon, 21 Sep 2015 21:40:19 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>
Subject: Re: [RFC PATCH 0/2] kvmclock: fix ABI breakage from
 PVCLOCK_COUNTS_FROM_ZERO.

> So either:
> 
> Proceed with guest solution:
> -> Make sure the overflow can't happen (and write down why not in the
> code). Don't assume a small delta between kvmclock values of vcpus.
> -> Handle stable -> non-stable kvmclock transition.
> -> kvmclock counts from zero should not depend on stable kvmclock
> (because nohz_full should work on hpet host systems).
> 
> Enable counts-from-zero on MSR_KVM_SYSTEM_TIME_NEW:
> -> Figure out whats wrong with different kvmclock values on hotplug, 
> and fix it.

Find data which allows you to differentiate between hotplug of pCPU-0
and system initialization.

Easy one: whether MSR_KVM_SYSTEM_TIME_NEW contains valid data (that is
kvmclock is enabled) for vCPUs other than vCPU-0.

This can't be the case on system initialization (otherwise the host will
be corrupting guest memory), and must be the case when hotplugging
vCPU-0.

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