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AJvYcCWxU53xU3MN8svP2oAUFztyl21v52So4gUl59mTGAxWwANQG/P1WT3KODhN6/6E14zbfqOASMd/LXSECd1nvcAVaNq28pZXPfNu X-Gm-Message-State: AOJu0YzEAoR2SXdkvzrmUitorjrxNDCnduK1WYQBUha5DHNVkohzrdyR 8prlmnSx8QtwOVs8mwpUmcW1xDgUd59HWP0eaqKAk3H0nEyW0tLY2qrtQgjYSqswF8biNSqMNNo xHg== X-Google-Smtp-Source: AGHT+IEyX+BvqryxMRr9G0bMzgAJHpudMNKKmYw7eyQeJ8a3uw3AhkQsT+dsqu7wlsIWbs8crlNnEDE965o= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6a02:451:b0:6e9:66a5:408d with SMTP id 41be03b00d2f7-6fae27627b0mr6025a12.1.1718227144135; Wed, 12 Jun 2024 14:19:04 -0700 (PDT) Date: Wed, 12 Jun 2024 14:19:02 -0700 In-Reply-To: <20240207172646.3981-7-xin3.li@intel.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240207172646.3981-1-xin3.li@intel.com> <20240207172646.3981-7-xin3.li@intel.com> Message-ID: Subject: Re: [PATCH v2 06/25] KVM: VMX: Defer enabling FRED MSRs save/load until after set CPUID From: Sean Christopherson To: Xin Li 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 Content-Type: text/plain; charset="us-ascii" 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.