linux-kernel - Re: [PATCH v3 07/12] KVM: X86: Add functions that calculate L2's TSC offset and multiplier

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Date:   Mon, 24 May 2021 20:51:09 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Ilias Stamatis <ilstam@...zon.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, zamsden@...il.com,
        mtosatti@...hat.com, dwmw@...zon.co.uk
Subject: Re: [PATCH v3 07/12] KVM: X86: Add functions that calculate L2's
 TSC offset and multiplier

On Fri, 2021-05-21 at 11:24 +0100, Ilias Stamatis wrote:
> When L2 is entered we need to "merge" the TSC multiplier and TSC offset
> values of 01 and 12 together.
> 
> The merging is done using the following equations:
>   offset_02 = ((offset_01 * mult_12) >> shift_bits) + offset_12
>   mult_02 = (mult_01 * mult_12) >> shift_bits
> 
> Where shift_bits is kvm_tsc_scaling_ratio_frac_bits.
> 
> Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/x86.c              | 25 +++++++++++++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0f2cf5d1240c..aaf756442ed1 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1792,6 +1792,8 @@ static inline bool kvm_is_supported_user_return_msr(u32 msr)
>  
>  u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc, u64 ratio);
>  u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc);
> +u64 kvm_calc_nested_tsc_offset(u64 l1_offset, u64 l2_offset, u64 l2_multiplier);
> +u64 kvm_calc_nested_tsc_multiplier(u64 l1_multiplier, u64 l2_multiplier);
>  
>  unsigned long kvm_get_linear_rip(struct kvm_vcpu *vcpu);
>  bool kvm_is_linear_rip(struct kvm_vcpu *vcpu, unsigned long linear_rip);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fdcb4f46a003..04abaacb9cfc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2334,6 +2334,31 @@ u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
>  }
>  EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
>  
> +u64 kvm_calc_nested_tsc_offset(u64 l1_offset, u64 l2_offset, u64 l2_multiplier)
> +{
> +	u64 nested_offset;
> +
> +	if (l2_multiplier == kvm_default_tsc_scaling_ratio)
> +		nested_offset = l1_offset;
> +	else
> +		nested_offset = mul_s64_u64_shr((s64) l1_offset, l2_multiplier,
> +						kvm_tsc_scaling_ratio_frac_bits);
> +
> +	nested_offset += l2_offset;
> +	return nested_offset;
> +}
> +EXPORT_SYMBOL_GPL(kvm_calc_nested_tsc_offset);
Looks OK.

> +
> +u64 kvm_calc_nested_tsc_multiplier(u64 l1_multiplier, u64 l2_multiplier)
> +{
> +	if (l2_multiplier != kvm_default_tsc_scaling_ratio)
> +		return mul_u64_u64_shr(l1_multiplier, l2_multiplier,
> +				       kvm_tsc_scaling_ratio_frac_bits);
> +
> +	return l1_multiplier;
> +}
> +EXPORT_SYMBOL_GPL(kvm_calc_nested_tsc_multiplier);
Looks OK as well.


> +
>  static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>  {
>  	vcpu->arch.l1_tsc_offset = offset;


Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky