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Date: Wed, 12 Jun 2024 14:19:02 -0700
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Subject: Re: [PATCH v2 06/25] KVM: VMX: Defer enabling FRED MSRs save/load
 until after set CPUID
From: Sean Christopherson <seanjc@google.com>
To: Xin Li <xin3.li@intel.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, 
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, 
	pbonzini@redhat.com, corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, 
	bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, 
	shuah@kernel.org, vkuznets@redhat.com, peterz@infradead.org, 
	ravi.v.shankar@intel.com, xin@zytor.com
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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.