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

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Date:	Fri, 18 Sep 2015 17:54:30 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>
Subject: [PATCH 2/2] Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR"

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

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