linux-kernel - [PATCH v4 06/19] KVM: VMX: Set FRED MSR interception

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Message-ID: <20250328171205.2029296-7-xin@zytor.com>
Date: Fri, 28 Mar 2025 10:11:52 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: pbonzini@...hat.com, seanjc@...gle.com, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        andrew.cooper3@...rix.com, luto@...nel.org, peterz@...radead.org,
        chao.gao@...el.com, xin3.li@...el.com
Subject: [PATCH v4 06/19] KVM: VMX: Set FRED MSR interception

From: Xin Li <xin3.li@...el.com>

Add FRED MSRs to the VMX passthrough MSR list and set FRED MSRs
interception.

8 FRED MSRs, i.e., MSR_IA32_FRED_RSP[123], MSR_IA32_FRED_STKLVLS,
MSR_IA32_FRED_SSP[123] and MSR_IA32_FRED_CONFIG, are all safe to
be passthrough, because they all have a pair of corresponding host
and guest VMCS fields.

Both MSR_IA32_FRED_RSP0 and MSR_IA32_FRED_SSP0 are dedicated for
userspace event delivery only, IOW they are NOT used in any kernel
event delivery and the execution of ERETS.  Thus KVM can run safely
with guest values in the 2 MSRs.  As a result, save and restore of
their guest values are deferred until vCPU context switch and their
host values are restored upon host returning to userspace.

Signed-off-by: Xin Li <xin3.li@...el.com>
Signed-off-by: Xin Li (Intel) <xin@...or.com>
Tested-by: Shan Kang <shan.kang@...el.com>
---
 arch/x86/kvm/vmx/vmx.c | 40 ++++++++++++++++++++++++++++++++++++++++
 arch/x86/kvm/vmx/vmx.h |  2 +-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ac6aa2d091c3..236fe5428a74 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -176,6 +176,16 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
 	MSR_FS_BASE,
 	MSR_GS_BASE,
 	MSR_KERNEL_GS_BASE,
+	MSR_IA32_FRED_RSP0,
+	MSR_IA32_FRED_RSP1,
+	MSR_IA32_FRED_RSP2,
+	MSR_IA32_FRED_RSP3,
+	MSR_IA32_FRED_STKLVLS,
+	MSR_IA32_FRED_SSP1,
+	MSR_IA32_FRED_SSP2,
+	MSR_IA32_FRED_SSP3,
+	MSR_IA32_FRED_CONFIG,
+	MSR_IA32_FRED_SSP0,		/* Should be added through CET */
 	MSR_IA32_XFD,
 	MSR_IA32_XFD_ERR,
 #endif
@@ -7935,6 +7945,34 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
 		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
 }
 
+static void vmx_set_intercept_for_fred_msr(struct kvm_vcpu *vcpu)
+{
+	bool flag = !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);
+
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP1, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP2, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP3, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_STKLVLS, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP1, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP2, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP3, MSR_TYPE_RW, flag);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_CONFIG, MSR_TYPE_RW, flag);
+
+	/*
+	 * IA32_FRED_RSP0 and IA32_PL0_SSP (a.k.a. IA32_FRED_SSP0) are only used
+	 * for delivering events when running userspace, while KVM always runs in
+	 * kernel mode (the CPL is always 0 after any VM exit), thus KVM can run
+	 * safely with guest IA32_FRED_RSP0 and IA32_PL0_SSP.
+	 *
+	 * As a result, no need to intercept IA32_FRED_RSP0 and IA32_PL0_SSP.
+	 *
+	 * Note, save and restore of IA32_PL0_SSP belong to CET supervisor context
+	 * management no matter whether FRED is enabled or not.  So leave its
+	 * state management to CET code.
+	 */
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP0, MSR_TYPE_RW, flag);
+}
+
 void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -8007,6 +8045,8 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 
 	/* Refresh #PF interception to account for MAXPHYADDR changes. */
 	vmx_update_exception_bitmap(vcpu);
+
+	vmx_set_intercept_for_fred_msr(vcpu);
 }
 
 static __init u64 vmx_get_perf_capabilities(void)
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index d53904db5d1a..f48791cf6aa6 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -356,7 +356,7 @@ struct vcpu_vmx {
 	struct lbr_desc lbr_desc;
 
 	/* Save desired MSR intercept (read: pass-through) state */
-#define MAX_POSSIBLE_PASSTHROUGH_MSRS	16
+#define MAX_POSSIBLE_PASSTHROUGH_MSRS	26
 	struct {
 		DECLARE_BITMAP(read, MAX_POSSIBLE_PASSTHROUGH_MSRS);
 		DECLARE_BITMAP(write, MAX_POSSIBLE_PASSTHROUGH_MSRS);
-- 
2.48.1