Re: [PATCH 2/2] Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR"

[Paolo Bonzini <pbonzini@redhat.com>]

On 22/09/2015 21:01, Marcelo Tosatti wrote:
> On Fri, Sep 18, 2015 at 05:54:30PM +0200, Radim Krčmář wrote:
>> Shifting pvclock_vcpu_time_info.system_time on write to KVM system time
>> MSR is a change of ABI.  Probably only 2.6.16 based SLES 10 breaks due
>> to its custom enhancements to kvmclock, but KVM never declared the MSR
>> only for one-shot initialization.  (Doc says that only one write is
>> needed.)
>>
>> This reverts commit b7e60c5aedd2b63f16ef06fde4f81ca032211bc5.
>> And adds a note to the definition of PVCLOCK_COUNTS_FROM_ZERO.
>>
>> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
>> ---
>>  arch/x86/include/asm/pvclock-abi.h | 1 +
>>  arch/x86/kvm/x86.c                 | 4 ----
>>  2 files changed, 1 insertion(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/pvclock-abi.h b/arch/x86/include/asm/pvclock-abi.h
>> index 655e07a48f6c..67f08230103a 100644
>> --- a/arch/x86/include/asm/pvclock-abi.h
>> +++ b/arch/x86/include/asm/pvclock-abi.h
>> @@ -41,6 +41,7 @@ struct pvclock_wall_clock {
>>  
>>  #define PVCLOCK_TSC_STABLE_BIT	(1 << 0)
>>  #define PVCLOCK_GUEST_STOPPED	(1 << 1)
>> +/* PVCLOCK_COUNTS_FROM_ZERO broke ABI and can't be used anymore. */
>>  #define PVCLOCK_COUNTS_FROM_ZERO (1 << 2)
>>  #endif /* __ASSEMBLY__ */
>>  #endif /* _ASM_X86_PVCLOCK_ABI_H */
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 18d59b584dee..34d33f4757d2 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1707,8 +1707,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>>  		vcpu->pvclock_set_guest_stopped_request = false;
>>  	}
>>  
>> -	pvclock_flags |= PVCLOCK_COUNTS_FROM_ZERO;
>> -
>>  	/* If the host uses TSC clocksource, then it is stable */
>>  	if (use_master_clock)
>>  		pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
>> @@ -2006,8 +2004,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>  					&vcpu->requests);
>>  
>>  			ka->boot_vcpu_runs_old_kvmclock = tmp;
>> -
>> -			ka->kvmclock_offset = -get_kernel_ns();
>>  		}
>>  
>>  		vcpu->arch.time = data;
>> -- 
>> 2.5.2
> 
> ACK

So I suppose you changed your mind :) but can you explain the reasoning?

Paolo