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

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Date:	Sun, 20 Sep 2015 19:57:42 -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: [RFC PATCH 0/2] kvmclock: fix ABI breakage from
 PVCLOCK_COUNTS_FROM_ZERO.

On Fri, Sep 18, 2015 at 05:54:28PM +0200, 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.

The justification to disable PVCLOCK_COUNTS_FROM_ZERO is because you
haven't explored potential problems or tested it? Sorry can't parse 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)

Please describe why SLES 10 breaks in detail: the state of the guest and
the host before the patch, the state of the guest and host after the
patch.

What does SLES10 expect?
Is it counting from zero that breaks SLES10? 

> 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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