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Date: Tue, 18 May 2021 23:04:14 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Ilias Stamatis <ilstam@...zon.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
pbonzini@...hat.com, mlevitsk@...hat.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 v2 03/10] KVM: X86: Add kvm_scale_tsc_l1() and
kvm_compute_tsc_offset_l1()
On Wed, May 12, 2021, Ilias Stamatis wrote:
> The existing kvm_scale_tsc() scales the TSC using the current TSC
> scaling ratio. That used to be the same as L1's scaling ratio but now
> with nested TSC scaling support it is no longer the case.
>
> This patch adds a new kvm_scale_tsc_l1() function that scales the TSC
> using L1's scaling ratio. The existing kvm_scale_tsc() can still be used
> for scaling L2 TSC values.
>
> Additionally, this patch renames the kvm_compute_tsc_offset() function
> to kvm_compute_tsc_offset_l1() and has the function treat its TSC
> argument as an L1 TSC value. All existing code uses this function
> passing L1 values to it.
>
> Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/x86.c | 41 ++++++++++++++++++++++-----------
> 2 files changed, 29 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7dfc609eacd6..be59197e5eb7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1789,6 +1789,7 @@ static inline bool kvm_is_supported_user_return_msr(u32 msr)
> }
>
> u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc);
> +u64 kvm_scale_tsc_l1(struct kvm_vcpu *vcpu, u64 tsc);
> u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc);
I don't really care which version is used, but we should be consistent, i.e. choose
kvm_<action>_tsc_l1 or kvm_<action>_tsc_l1, not both. The easy choice is the
former since it's already there.
> unsigned long kvm_get_linear_rip(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 07cf5d7ece38..84af1af7a2cc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2319,18 +2319,30 @@ u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc)
> }
> EXPORT_SYMBOL_GPL(kvm_scale_tsc);
>
> -static u64 kvm_compute_tsc_offset(struct kvm_vcpu *vcpu, u64 target_tsc)
> +u64 kvm_scale_tsc_l1(struct kvm_vcpu *vcpu, u64 tsc)
> +{
> + u64 _tsc = tsc;
> + u64 ratio = vcpu->arch.l1_tsc_scaling_ratio;
> +
> + if (ratio != kvm_default_tsc_scaling_ratio)
> + _tsc = __scale_tsc(ratio, tsc);
> +
> + return _tsc;
> +}
Just make the ratio a param. This is complete copy+paste of kvm_scale_tsc(),
with 3 characters added. And all of the callers are already in an L1-specific
function or have L1 vs. L2 awareness. IMO, that makes the code less magical, too,
as I don't have to dive into a helper to see that it reads l1_tsc_scaling_ratio
versus tsc_scaling_ratio.
> +EXPORT_SYMBOL_GPL(kvm_scale_tsc_l1);
> +
> +static u64 kvm_compute_tsc_offset_l1(struct kvm_vcpu *vcpu, u64 target_tsc)
> {
> u64 tsc;
>
> - tsc = kvm_scale_tsc(vcpu, rdtsc());
> + tsc = kvm_scale_tsc_l1(vcpu, rdtsc());
>
> return target_tsc - tsc;
> }
>
> u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc)
> {
> - return vcpu->arch.l1_tsc_offset + kvm_scale_tsc(vcpu, host_tsc);
> + return vcpu->arch.l1_tsc_offset + kvm_scale_tsc_l1(vcpu, host_tsc);
> }
> EXPORT_SYMBOL_GPL(kvm_read_l1_tsc);
>
> @@ -2363,7 +2375,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
> bool synchronizing = false;
>
> raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
> - offset = kvm_compute_tsc_offset(vcpu, data);
> + offset = kvm_compute_tsc_offset_l1(vcpu, data);
> ns = get_kvmclock_base_ns();
> elapsed = ns - kvm->arch.last_tsc_nsec;
>
> @@ -2402,7 +2414,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
> } else {
> u64 delta = nsec_to_cycles(vcpu, elapsed);
> data += delta;
> - offset = kvm_compute_tsc_offset(vcpu, data);
> + offset = kvm_compute_tsc_offset_l1(vcpu, data);
> }
> matched = true;
> already_matched = (vcpu->arch.this_tsc_generation == kvm->arch.cur_tsc_generation);
> @@ -2463,7 +2475,7 @@ static inline void adjust_tsc_offset_host(struct kvm_vcpu *vcpu, s64 adjustment)
> {
> if (vcpu->arch.l1_tsc_scaling_ratio != kvm_default_tsc_scaling_ratio)
> WARN_ON(adjustment < 0);
> - adjustment = kvm_scale_tsc(vcpu, (u64) adjustment);
> + adjustment = kvm_scale_tsc_l1(vcpu, (u64) adjustment);
> adjust_tsc_offset_guest(vcpu, adjustment);
> }
>
> @@ -2846,7 +2858,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
> /* With all the info we got, fill in the values */
>
> if (kvm_has_tsc_control)
> - tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);
> + tgt_tsc_khz = kvm_scale_tsc_l1(v, tgt_tsc_khz);
>
> if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
> kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
> @@ -3235,7 +3247,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> if (msr_info->host_initiated) {
> kvm_synchronize_tsc(vcpu, data);
> } else {
> - u64 adj = kvm_compute_tsc_offset(vcpu, data) - vcpu->arch.l1_tsc_offset;
> + u64 adj = kvm_compute_tsc_offset_l1(vcpu, data) - vcpu->arch.l1_tsc_offset;
> adjust_tsc_offset_guest(vcpu, adj);
> vcpu->arch.ia32_tsc_adjust_msr += adj;
> }
> @@ -3537,10 +3549,13 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> * return L1's TSC value to ensure backwards-compatible
> * behavior for migration.
> */
> - u64 tsc_offset = msr_info->host_initiated ? vcpu->arch.l1_tsc_offset :
> - vcpu->arch.tsc_offset;
> -
> - msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) + tsc_offset;
> + if (msr_info->host_initiated) {
Unnecessary curly braces.
> + msr_info->data = kvm_scale_tsc_l1(vcpu, rdtsc()) +
> + vcpu->arch.l1_tsc_offset;
> + } else {
> + msr_info->data = kvm_scale_tsc(vcpu, rdtsc()) +
> + vcpu->arch.tsc_offset;
> + }
> break;
> }
> case MSR_MTRRcap:
> @@ -4123,7 +4138,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> mark_tsc_unstable("KVM discovered backwards TSC");
>
> if (kvm_check_tsc_unstable()) {
> - u64 offset = kvm_compute_tsc_offset(vcpu,
> + u64 offset = kvm_compute_tsc_offset_l1(vcpu,
> vcpu->arch.last_guest_tsc);
> kvm_vcpu_write_tsc_offset(vcpu, offset);
> vcpu->arch.tsc_catchup = 1;
> --
> 2.17.1
>
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