linux-kernel - [PATCH v2 1/7] KVM: VMX: Restore host's MSR_IA32_RTIT

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Date:   Fri, 27 Aug 2021 15:02:43 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/7] KVM: VMX: Restore host's MSR_IA32_RTIT_CTL when it's not zero

A minor optimization to WRMSR MSR_IA32_RTIT_CTL when necessary.

Opportunistically refine the comment to call out that KVM requires
VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest.

Reviewed-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
---
Changes in v2:
 - Refine comments
---
 arch/x86/kvm/vmx/vmx.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fada1055f325..5535a86aea37 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1074,8 +1074,12 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
 		pt_load_msr(&vmx->pt_desc.host, vmx->pt_desc.addr_range);
 	}
 
-	/* Reload host state (IA32_RTIT_CTL will be cleared on VM exit). */
-	wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
+	/*
+	 * KVM requires VM_EXIT_CLEAR_IA32_RTIT_CTL to expose PT to the guest,
+	 * i.e. RTIT_CTL is always cleared on VM-Exit.  Restore it if necessary.
+	 */
+	if (vmx->pt_desc.host.ctl)
+		wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
 }
 
 void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
-- 
2.27.0