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

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Date: Thu, 13 Jun 2024 03:31:07 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "linux-kselftest@...r.kernel.org"
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	<ravi.v.shankar@...el.com>, "xin@...or.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.

No performance measurement yet.  Will make the change.

> 
> 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.

Simple rule.

Thanks!
    Xin