linux-kernel - Re: [PATCH v2 06/25] KVM: VMX: Defer enabling FRED MSRs save/load until after set CPUID

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Message-ID: <ZmoQxjksn5bWhlBA@google.com>
Date: Wed, 12 Jun 2024 14:19:02 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin3.li@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	pbonzini@...hat.com, corbet@....net, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	shuah@...nel.org, vkuznets@...hat.com, peterz@...radead.org, 
	ravi.v.shankar@...el.com, xin@...or.com
Subject: Re: [PATCH v2 06/25] KVM: VMX: Defer enabling FRED MSRs save/load
 until after set CPUID

On Wed, Feb 07, 2024, Xin Li wrote:
> Clear FRED VM entry/exit controls when initializing a vCPU, and set
> these controls only if FRED is enumerated after set CPUID.
> 
> FRED VM entry/exit controls need to be set to establish context
> sufficient to support FRED event delivery immediately after VM entry
> and exit.  However it is not required to save/load FRED MSRs for
> a non-FRED guest, which aren't supposed to access FRED MSRs.

Does this actually provide a measurable performance boost?  If not, just do the
unnecessary load/store on entry/exit.

Generally speaking, the only time KVM dynamically toggles entry/exit controls is
when KVM wants to run the guest with a host value, e.g. with the host's
PERF_GLOBAL_CTRL.