Re: [RFC PATCH 0/2] kvmclock: fix ABI breakage from PVCLOCK_COUNTS_FROM_ZERO.

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>, linux-kernel@vger.kernel.org
Cc: kvm@vger.kernel.org, Marcelo Tosatti <mtosatti@redhat.com>,
	Luiz Capitulino <lcapitulino@redhat.com>
Subject: Re: [RFC PATCH 0/2] kvmclock: fix ABI breakage from PVCLOCK_COUNTS_FROM_ZERO.
Date: Mon, 28 Sep 2015 13:05:58 +0200	[thread overview]
Message-ID: <56091F16.5050503@redhat.com> (raw)
In-Reply-To: <1442591670-5216-1-git-send-email-rkrcmar@redhat.com>


On 18/09/2015 17:54, Radim Krčmář wrote:
> 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)

Thanks,

applying 2/2 for 4.4 and 1/2 for 4.3.

Paolo

> 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(-)
>