linux-kernel - Re: [PATCH 0/2] KVM: x86: do not mix raw and monotonic clocks in kvmclock

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Date:   Fri, 24 Jan 2020 17:36:30 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 0/2] KVM: x86: do not mix raw and monotonic clocks in
 kvmclock

On Wed, Jan 22, 2020 at 03:22:31PM +0100, Paolo Bonzini wrote:
> Commit 53fafdbb8b21f ("KVM: x86: switch KVMCLOCK base to monotonic raw
> clock") changed kvmclock to use tkr_raw instead of tkr_mono.  However,
> the default kvmclock_offset for the VM was still based on the monotonic
> clock and, if the raw clock drifted enough from the monotonic clock,
> this could cause a negative system_time to be written to the guest's
> struct pvclock.  RHEL5 does not like it and (if it boots fast enough to
> observe a negative time value) it hangs.
> 
> This series fixes the issue by using the raw clock everywhere.
> 
> (And this, ladies and gentlemen, is why I was not applying patches to
> the KVM tree.  I saw this before Christmas and could only reproduce it
> today, since it requires almost 2 weeks of uptime to reproduce on my
> machine.  Of course, once you have the reproducer the fix is relatively
> easy to come up with).
> 
> Paolo
> 
> Paolo Bonzini (2):
>   KVM: x86: reorganize pvclock_gtod_data members
>   KVM: x86: use raw clock values consistently
> 
>  arch/x86/kvm/x86.c | 67 ++++++++++++++++++++++++++++--------------------------
>  1 file changed, 35 insertions(+), 32 deletions(-)
> 
> -- 
> 1.8.3.1

Reviewed-by: Marcelo Tosatti <mtosatti@...hat.com>

BTW, should switch both masterclock and non-masterclock cases
to raw clock base. Do you see any problem with that? 

Using the same reasoning as raw clock for master, ntpd in 
the guest should correct the difference.

Could probably simplify things.