linux-kernel - [RFC PATCH 0/2] kvmclock: fix ABI breakage from PVCLOCK_COUNTS_FROM

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Date:	Fri, 18 Sep 2015 17:54:28 +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: [RFC PATCH 0/2] kvmclock: fix ABI breakage from PVCLOCK_COUNTS_FROM_ZERO.

This patch series will be disabling PVCLOCK_COUNTS_FROM_ZERO flag and is
RFC because I haven't explored many potential problems or tested it.

[1/2] uses a different algorithm in the guest to start counting from 0.
[2/2] stops exposing PVCLOCK_COUNTS_FROM_ZERO in the hypervisor.

A viable alternative would be to implement opt-in features in kvm clock.

And because we probably only broke one old user (the infamous SLES 10), a
workaround like this is also possible: (but I'd rather not do that)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a60bdbccff51..ae9049248aaf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2007,7 +2007,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 
 			ka->boot_vcpu_runs_old_kvmclock = tmp;
 
-			ka->kvmclock_offset = -get_kernel_ns();
+			if (!ka->boot_vcpu_runs_old_kvmclock)
+				ka->kvmclock_offset = -get_kernel_ns();
 		}
 
 		vcpu->arch.time = data;


Radim Krčmář (2):
  x86: kvmclock: abolish PVCLOCK_COUNTS_FROM_ZERO
  Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock
    system MSR"

 arch/x86/include/asm/pvclock-abi.h |  1 +
 arch/x86/kernel/kvmclock.c         | 46 +++++++++++++++++++++++++++++---------
 arch/x86/kvm/x86.c                 |  4 ----
 3 files changed, 36 insertions(+), 15 deletions(-)

-- 
2.5.2

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