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

Maxim Levitsky
2021-05-24T17:51:24Z

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@amazon.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@redhat.com>

Best regards,
Maxim Levitsky