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

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Message-ID: <20150922190133.GB23748@amt.cnet>
Date:	Tue, 22 Sep 2015 16:01:33 -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: [PATCH 2/2] Revert "KVM: x86: zero kvmclock_offset when vcpu0
 initializes kvmclock system MSR"

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@...hat.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

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