From: Sean Christopherson <seanjc@google.com>
To: "Stamatis, Ilias" <ilstam@amazon.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"jmattson@google.com" <jmattson@google.com>,
"Woodhouse, David" <dwmw@amazon.co.uk>,
"vkuznets@redhat.com" <vkuznets@redhat.com>,
"mtosatti@redhat.com" <mtosatti@redhat.com>,
"joro@8bytes.org" <joro@8bytes.org>,
"zamsden@gmail.com" <zamsden@gmail.com>,
"mlevitsk@redhat.com" <mlevitsk@redhat.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"wanpengli@tencent.com" <wanpengli@tencent.com>
Subject: Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc_multiplier()
Date: Tue, 25 May 2021 23:35:03 +0000 [thread overview]
Message-ID: <YK2Jp2tZzIkik142@google.com> (raw)
In-Reply-To: <6d18b842e1ab946da2e0ebfae79fc51c3193802a.camel@amazon.com>
On Tue, May 25, 2021, Stamatis, Ilias wrote:
> Hmm, this patch actually still removes the caching and introduces a small
> performance overhead. For example if neither L1 nor L2 are scaled it will
> still do a vmwrite for every L2 entry/write.
True, but there is an ocean of difference between the relative performance of
vmx_vcpu_load_vmcs() and a nested transition. vmx_vcpu_load_vmcs() is also
called much more frequently.
> So do we want to get rid of decache_tsc_multiplier() but keep
> vmx->current_tsc_ratio and do the check inside write_tsc_multiplier()? Or
> alternatively delete vmx->current_tsc_ratio too and have
> write_tsc_multiplier() receive 2 parameters, one of the old multiplier and
> one of the new?
My vote is to kill it, eat the barely-noticeable perf hit on nVMX, and tackle
the aggressive VMCS shadowing in a separate series.
next prev parent reply other threads:[~2021-05-25 23:35 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
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 [this message]
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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