linux-kernel - Re: [PATCH v2 13/15] KVM: x86: Kill cur_tsc

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Date: Fri, 10 May 2024 17:03:28 +0800
From: Chenyi Qiang <chenyi.qiang@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, <kvm@...r.kernel.org>
CC: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
	Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
	<bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Paul Durrant <paul@....org>, Shuah Khan
	<shuah@...nel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>, "Oliver
 Upton" <oliver.upton@...ux.dev>, Marcelo Tosatti <mtosatti@...hat.com>,
	<jalliste@...zon.co.uk>, <sveith@...zon.de>, <zide.chen@...el.com>, "Dongli
 Zhang" <dongli.zhang@...cle.com>
Subject: Re: [PATCH v2 13/15] KVM: x86: Kill cur_tsc_{nsec,offset,write}
 fields



On 4/27/2024 7:05 PM, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> These pointlessly duplicate of the last_tsc_{nsec,offset,write} values.
> 
> The only place they were used was where the TSC is stable and a new vCPU
> is being synchronized to the previous setting, in which case the 'last_'
> value is definitely identical.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  arch/x86/include/asm/kvm_host.h | 3 ---
>  arch/x86/kvm/x86.c              | 9 ++-------
>  2 files changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b01c1d000fff..7d06f389a607 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1354,9 +1354,6 @@ struct kvm_arch {
>  	u32 last_tsc_khz;
>  	u64 last_tsc_offset;
>  	u64 last_tsc_scaling_ratio;
> -	u64 cur_tsc_nsec;
> -	u64 cur_tsc_write;
> -	u64 cur_tsc_offset;
>  	u64 cur_tsc_generation;
>  	int nr_vcpus_matched_tsc;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6ec43f39bdb0..92e81bfca25a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2737,9 +2737,6 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
>  		 * These values are tracked in kvm->arch.cur_xxx variables.
>  		 */
>  		kvm->arch.cur_tsc_generation++;
> -		kvm->arch.cur_tsc_nsec = ns;
> -		kvm->arch.cur_tsc_write = tsc;
> -		kvm->arch.cur_tsc_offset = offset;
>  		kvm->arch.nr_vcpus_matched_tsc = 0;
>  	} else if (vcpu->arch.this_tsc_generation != kvm->arch.cur_tsc_generation) {
>  		kvm->arch.nr_vcpus_matched_tsc++;
> @@ -2747,8 +2744,6 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
>  
>  	/* Keep track of which generation this VCPU has synchronized to */
>  	vcpu->arch.this_tsc_generation = kvm->arch.cur_tsc_generation;
> -	vcpu->arch.this_tsc_nsec = kvm->arch.cur_tsc_nsec;
> -	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;

Do we need to track vcpu->arch.this_tsc_nsec/this_tsc_write? At least
they are still used in compute_guest_tsc() to calculate the guest tsc.

>  
>  	kvm_track_tsc_matching(vcpu);
>  }
> @@ -2825,8 +2820,8 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
>  		data = kvm->arch.last_tsc_write;
>  
>  		if (!kvm_check_tsc_unstable()) {
> -			offset = kvm->arch.cur_tsc_offset;
> -			ns = kvm->arch.cur_tsc_nsec;
> +			offset = kvm->arch.last_tsc_offset;
> +			ns = kvm->arch.last_tsc_nsec;
>  		} else {
>  			/*
>  			 * ... unless the TSC is unstable and has to be