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

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Date:   Sat, 25 Jan 2020 10:42:40 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...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 24/01/20 21:36, Marcelo Tosatti wrote:
> 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? 

Indeed, that makes sense as a kind of unification of the code.  Together
with adding pvclock_gtod support for 32-bit, it should be easy.

Paolo