linux-kernel - [PATCH v13 09/21] KVM: x86: Enable guest SSP read/write interface with new uAPIs

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Message-ID: <20250821133132.72322-10-chao.gao@intel.com>
Date: Thu, 21 Aug 2025 06:30:43 -0700
From: Chao Gao <chao.gao@...el.com>
To: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: chao.gao@...el.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com,
	john.allen@....com,
	mingo@...hat.com,
	minipli@...ecurity.net,
	mlevitsk@...hat.com,
	pbonzini@...hat.com,
	rick.p.edgecombe@...el.com,
	seanjc@...gle.com,
	tglx@...utronix.de,
	weijiang.yang@...el.com,
	x86@...nel.org,
	xin@...or.com
Subject: [PATCH v13 09/21] KVM: x86: Enable guest SSP read/write interface with new uAPIs

From: Yang Weijiang <weijiang.yang@...el.com>

Enable guest shadow stack pointer(SSP) access interface with new uAPIs.
CET guest SSP is HW register which has corresponding VMCS field to save
/restore guest values when VM-{Exit,Entry} happens. KVM handles SSP as
a synthetic MSR for userspace access.

Use a translation helper to set up mapping for SSP synthetic index and
KVM-internal MSR index so that userspace doesn't need to take care of
KVM's management for synthetic MSRs and avoid conflicts.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
Tested-by: Mathias Krause <minipli@...ecurity.net>
Tested-by: John Allen <john.allen@....com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
---
 arch/x86/include/uapi/asm/kvm.h |  3 +++
 arch/x86/kvm/x86.c              | 10 +++++++++-
 arch/x86/kvm/x86.h              | 10 ++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 969a63e73190..478d9b63a9db 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -432,6 +432,9 @@ struct kvm_xcrs {
 #define KVM_X86_REG_SYNTHETIC_MSR(index)			\
 	KVM_X86_REG_ENCODE(KVM_X86_REG_TYPE_SYNTHETIC_MSR, index)
 
+/* KVM synthetic MSR index staring from 0 */
+#define KVM_SYNTHETIC_GUEST_SSP 0
+
 #define KVM_SYNC_X86_REGS      (1UL << 0)
 #define KVM_SYNC_X86_SREGS     (1UL << 1)
 #define KVM_SYNC_X86_EVENTS    (1UL << 2)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 61e008be172d..a48e53b98c95 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5998,7 +5998,15 @@ struct kvm_x86_reg_id {
 
 static int kvm_translate_synthetic_msr(struct kvm_x86_reg_id *reg)
 {
-	return -EINVAL;
+	switch (reg->index) {
+	case KVM_SYNTHETIC_GUEST_SSP:
+		reg->type = KVM_X86_REG_TYPE_MSR;
+		reg->index = MSR_KVM_INTERNAL_GUEST_SSP;
+		break;
+	default:
+		return -EINVAL;
+	}
+	return 0;
 }
 
 long kvm_arch_vcpu_ioctl(struct file *filp,
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index d90f1009ac10..0d11115dcd2a 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -101,6 +101,16 @@ do {											\
 #define KVM_SVM_DEFAULT_PLE_WINDOW_MAX	USHRT_MAX
 #define KVM_SVM_DEFAULT_PLE_WINDOW	3000
 
+/*
+ * KVM's internal, non-ABI indices for synthetic MSRs. The values themselves
+ * are arbitrary and have no meaning, the only requirement is that they don't
+ * conflict with "real" MSRs that KVM supports. Use values at the upper end
+ * of KVM's reserved paravirtual MSR range to minimize churn, i.e. these values
+ * will be usable until KVM exhausts its supply of paravirtual MSR indices.
+ */
+
+#define MSR_KVM_INTERNAL_GUEST_SSP	0x4b564dff
+
 static inline unsigned int __grow_ple_window(unsigned int val,
 		unsigned int base, unsigned int modifier, unsigned int max)
 {
-- 
2.47.3