linux-kernel - [RFC PATCH 2/2] KVM: x86: Enable guest SSP read/write interface with new uAPIs

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Date: Thu,  9 May 2024 00:54:23 -0700
From: Yang Weijiang <weijiang.yang@...el.com>
To: seanjc@...gle.com,
	pbonzini@...hat.com,
	mlevitsk@...hat.com,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Yang Weijiang <weijiang.yang@...el.com>
Subject: [RFC PATCH 2/2] KVM: x86: Enable guest SSP read/write interface with new uAPIs

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>
---
 arch/x86/include/uapi/asm/kvm.h |  3 +++
 arch/x86/kvm/x86.c              |  7 +++++++
 arch/x86/kvm/x86.h              | 10 ++++++++++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index ca2a47a85fa1..81c8d9ea2e58 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -420,6 +420,9 @@ struct kvm_x86_reg_id {
 	__u16 rsvd16;
 };
 
+/* KVM synthetic MSR index staring from 0 */
+#define MSR_KVM_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 d0054c52f24b..a970bd26ce2c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5886,6 +5886,13 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 
 static int kvm_translate_synthetic_msr(u32 *index)
 {
+	switch (*index) {
+	case MSR_KVM_GUEST_SSP:
+		*index = MSR_KVM_INTERNAL_GUEST_SSP;
+		break;
+	default:
+		return -EINVAL;
+	}
 	return 0;
 }
 
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index a8b71803777b..6ac86a75aedc 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -57,6 +57,16 @@ void kvm_spurious_fault(void);
 #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 uppper 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)
 {
-- 
2.43.0