From: Maxim Levitsky <mlevitsk@redhat.com>
To: Ilias Stamatis <ilstam@amazon.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
pbonzini@redhat.com
Cc: seanjc@google.com, vkuznets@redhat.com, wanpengli@tencent.com,
jmattson@google.com, joro@8bytes.org, zamsden@gmail.com,
mtosatti@redhat.com, dwmw@amazon.co.uk
Subject: Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc_multiplier()
Date: Mon, 24 May 2021 20:53:54 +0300 [thread overview]
Message-ID: <2b3bc8aff14a09c4ea4a1b648f750b5ffb1a15a0.camel@redhat.com> (raw)
In-Reply-To: <20210521102449.21505-10-ilstam@amazon.com>
On Fri, 2021-05-21 at 11:24 +0100, Ilias Stamatis wrote:
> The vmx->current_tsc_ratio field is redundant as
> vcpu->arch.tsc_scaling_ratio already tracks the current TSC scaling
> ratio. Removing this field makes decache_tsc_multiplier() an one-liner
> so remove that too and do a vmcs_write64() directly in order to be more
> consistent with surrounding code.
Not to mention that 'decache_tsc_multiplier' isn't a good name IMHO
for this....
>
> Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
> ---
> arch/x86/kvm/vmx/nested.c | 9 ++++-----
> arch/x86/kvm/vmx/vmx.c | 5 ++---
> arch/x86/kvm/vmx/vmx.h | 8 --------
> 3 files changed, 6 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 6058a65a6ede..239154d3e4e7 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2533,9 +2533,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> }
>
> vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> -
> if (kvm_has_tsc_control)
> - decache_tsc_multiplier(vmx);
> + vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
>
> nested_vmx_transition_tlb_flush(vcpu, vmcs12, true);
>
> @@ -4501,12 +4500,12 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
> vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
> vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
> vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> + if (kvm_has_tsc_control)
> + vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
> +
> if (vmx->nested.l1_tpr_threshold != -1)
> vmcs_write32(TPR_THRESHOLD, vmx->nested.l1_tpr_threshold);
>
> - if (kvm_has_tsc_control)
> - decache_tsc_multiplier(vmx);
> -
> if (vmx->nested.change_vmcs01_virtual_apic_mode) {
> vmx->nested.change_vmcs01_virtual_apic_mode = false;
> vmx_set_virtual_apic_mode(vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4b70431c2edd..7c52c697cfe3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1392,9 +1392,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
> }
>
> /* Setup TSC multiplier */
> - if (kvm_has_tsc_control &&
> - vmx->current_tsc_ratio != vcpu->arch.tsc_scaling_ratio)
> - decache_tsc_multiplier(vmx);
> + if (kvm_has_tsc_control)
> + vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
This might have an overhead of writing the TSC scaling ratio even if
it is unchanged. I haven't measured how expensive vmread/vmwrites are but
at least when nested, the vmreads/vmwrites can be very expensive (if they
cause a vmexit).
This is why I think the 'vmx->current_tsc_ratio' exists - to have
a cached value of TSC scale ratio to avoid either 'vmread'ing
or 'vmwrite'ing it without a need.
Best regards,
Maxim Levitsky
> }
>
> /*
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index aa97c82e3451..3eaa86a0ba3e 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -322,8 +322,6 @@ struct vcpu_vmx {
> /* apic deadline value in host tsc */
> u64 hv_deadline_tsc;
>
> - u64 current_tsc_ratio;
> -
> unsigned long host_debugctlmsr;
>
> /*
> @@ -532,12 +530,6 @@ static inline struct vmcs *alloc_vmcs(bool shadow)
> GFP_KERNEL_ACCOUNT);
> }
>
> -static inline void decache_tsc_multiplier(struct vcpu_vmx *vmx)
> -{
> - vmx->current_tsc_ratio = vmx->vcpu.arch.tsc_scaling_ratio;
> - vmcs_write64(TSC_MULTIPLIER, vmx->current_tsc_ratio);
> -}
> -
> static inline bool vmx_has_waitpkg(struct vcpu_vmx *vmx)
> {
> return vmx->secondary_exec_control &
next prev parent reply other threads:[~2021-05-24 17:54 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-21 10:24 [PATCH v3 00/12] KVM: Implement nested TSC scaling Ilias Stamatis
2021-05-21 10:24 ` [PATCH v3 01/12] math64.h: Add mul_s64_u64_shr() Ilias Stamatis
2021-05-24 17:49 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 02/12] KVM: X86: Store L1's TSC scaling ratio in 'struct kvm_vcpu_arch' Ilias Stamatis
2021-05-24 17:49 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 03/12] KVM: X86: Rename kvm_compute_tsc_offset() to kvm_compute_tsc_offset_l1() Ilias Stamatis
2021-05-24 14:21 ` Paolo Bonzini
2021-05-24 17:49 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 04/12] KVM: X86: Add a ratio parameter to kvm_scale_tsc() Ilias Stamatis
2021-05-24 14:23 ` Paolo Bonzini
2021-05-24 15:48 ` Sean Christopherson
2021-05-24 15:56 ` Paolo Bonzini
2021-05-24 17:50 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 05/12] KVM: VMX: Add a TSC multiplier field in VMCS12 Ilias Stamatis
2021-05-21 10:24 ` [PATCH v3 06/12] KVM: X86: Add functions for retrieving L2 TSC fields from common code Ilias Stamatis
2021-05-24 17:50 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 07/12] KVM: X86: Add functions that calculate L2's TSC offset and multiplier Ilias Stamatis
2021-05-24 17:51 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 08/12] KVM: X86: Move write_l1_tsc_offset() logic to common code and rename it Ilias Stamatis
2021-05-24 17:51 ` Maxim Levitsky
2021-05-21 10:24 ` [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc_multiplier() Ilias Stamatis
2021-05-24 17:53 ` Maxim Levitsky [this message]
2021-05-24 18:44 ` Sean Christopherson
2021-05-25 10:41 ` Stamatis, Ilias
2021-05-25 15:58 ` Sean Christopherson
2021-05-25 16:15 ` Paolo Bonzini
2021-05-25 16:34 ` Sean Christopherson
2021-05-25 17:34 ` Paolo Bonzini
2021-05-25 18:21 ` Sean Christopherson
2021-05-25 18:52 ` Stamatis, Ilias
2021-05-25 19:25 ` Stamatis, Ilias
2021-05-25 23:35 ` Sean Christopherson
2021-05-21 10:24 ` [PATCH v3 10/12] KVM: VMX: Set the TSC offset and multiplier on nested entry and exit Ilias Stamatis
2021-05-24 17:54 ` Maxim Levitsky
2021-05-25 16:05 ` Sean Christopherson
2021-05-21 10:24 ` [PATCH v3 11/12] KVM: VMX: Expose TSC scaling to L2 Ilias Stamatis
2021-05-21 10:24 ` [PATCH v3 12/12] KVM: selftests: x86: Add vmx_nested_tsc_scaling_test Ilias Stamatis
2021-05-24 17:55 ` Maxim Levitsky
2021-05-24 15:37 ` [PATCH v3 00/12] KVM: Implement nested TSC scaling Paolo Bonzini
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