linux-kernel - Re: [PATCH 02/12] KVM: x86: Add a common TSC scaling ratio field in kvm_vcpu

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Message-ID: <20151006015404.GC3798@hzzhang-OptiPlex-9020.sh.intel.com>
Date:	Tue, 6 Oct 2015 09:54:04 +0800
From:	Haozhong Zhang <haozhong.zhang@...el.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	kvm@...r.kernel.org, Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Joerg Roedel <joro@...tes.org>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Mihai Donțu <mdontu@...defender.com>,
	Andy Lutomirski <luto@...nel.org>,
	Kai Huang <kai.huang@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/12] KVM: x86: Add a common TSC scaling ratio field in
 kvm_vcpu_arch

On Mon, Oct 05, 2015 at 09:26:30PM +0200, Radim Krčmář wrote:
> 2015-09-28 13:38+0800, Haozhong Zhang:
> > This patch moves the field of TSC scaling ratio from the architecture
> > struct vcpu_svm to the common struct kvm_vcpu_arch.
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
> > ---
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > @@ -7080,6 +7080,9 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
> >  
> >  	vcpu = kvm_x86_ops->vcpu_create(kvm, id);
> >  
> > +	if (!IS_ERR(vcpu))
> > +		vcpu->arch.tsc_scaling_ratio = kvm_default_tsc_scaling_ratio;
> 
> This shouldn't be necessary, (and we can definitely do it without error
> checking later)
> 
>  kvm_arch_vcpu_create
>    (vmx|svm)_create_vcpu
>      kvm_vcpu_init
>        kvm_arch_vcpu_init
>          kvm_set_tsc_khz
> 
> sets vcpu->arch.tsc_scaling_ratio to something reasonable and SVM didn't
> overwrite that value.  (kvm_set_tsc_khz() only doesn't set the ration if
> this_tsc_khz == 0, which we could extend to be extra safe.)

Thanks Radim! I even didn't notice this path. I'll remove the ratio
setting in kvm_arch_vcpu_create(). In kvm_set_tsc_khz(), if
this_tsc_khz == 0, I'll make it set vcpu->arch.tsc_scaling_ratio to
kvm_default_tsc_scaling_ratio.

- Haozhong
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