linux-kernel - [PATCH v2 03/16] KVM: x86: Implement accessors for extended GPRs

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Message-ID: <20260112235408.168200-4-chang.seok.bae@intel.com>
Date: Mon, 12 Jan 2026 23:53:55 +0000
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	chao.gao@...el.com,
	chang.seok.bae@...el.com
Subject: [PATCH v2 03/16] KVM: x86: Implement accessors for extended GPRs

Add helpers to directly read and write EGPRs (R16–R31).

Unlike legacy GPRs, EGPRs are not cached in vcpu->arch.regs[]. Their
contents remain live in hardware. If preempted, the EGPR state is
preserved in the guest XSAVE buffer.

The Advanced Performance Extensions (APX) feature introduces EGPRs as an
XSAVE-managed state component. The new helpers access the registers
directly between kvm_fpu_get() and kvm_fpu_put().

Callers should ensure that EGPRs are enabled before using these helpers.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
---
V1 -> V2: Move _kvm_read_egpr()/_kvm_write_egpr() to x86.c (Paolo)
---
 arch/x86/kvm/x86.c | 70 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9857b4d319ed..edac2ec11e2f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1256,13 +1256,81 @@ static inline u64 kvm_guest_supported_xfd(struct kvm_vcpu *vcpu)
 #endif
 
 #ifdef CONFIG_KVM_APX
+/*
+ * Accessors for extended general-purpose registers. binutils >= 2.43 can
+ * recognize those register symbols.
+ */
+
+static void _kvm_read_egpr(int reg, unsigned long *data)
+{
+	/* mov %r16..%r31, %rax */
+	switch (reg) {
+	case __VCPU_XREG_R16: asm(".byte 0xd5, 0x48, 0x89, 0xc0" : "=a"(*data)); break;
+	case __VCPU_XREG_R17: asm(".byte 0xd5, 0x48, 0x89, 0xc8" : "=a"(*data)); break;
+	case __VCPU_XREG_R18: asm(".byte 0xd5, 0x48, 0x89, 0xd0" : "=a"(*data)); break;
+	case __VCPU_XREG_R19: asm(".byte 0xd5, 0x48, 0x89, 0xd8" : "=a"(*data)); break;
+	case __VCPU_XREG_R20: asm(".byte 0xd5, 0x48, 0x89, 0xe0" : "=a"(*data)); break;
+	case __VCPU_XREG_R21: asm(".byte 0xd5, 0x48, 0x89, 0xe8" : "=a"(*data)); break;
+	case __VCPU_XREG_R22: asm(".byte 0xd5, 0x48, 0x89, 0xf0" : "=a"(*data)); break;
+	case __VCPU_XREG_R23: asm(".byte 0xd5, 0x48, 0x89, 0xf8" : "=a"(*data)); break;
+	case __VCPU_XREG_R24: asm(".byte 0xd5, 0x4c, 0x89, 0xc0" : "=a"(*data)); break;
+	case __VCPU_XREG_R25: asm(".byte 0xd5, 0x4c, 0x89, 0xc8" : "=a"(*data)); break;
+	case __VCPU_XREG_R26: asm(".byte 0xd5, 0x4c, 0x89, 0xd0" : "=a"(*data)); break;
+	case __VCPU_XREG_R27: asm(".byte 0xd5, 0x4c, 0x89, 0xd8" : "=a"(*data)); break;
+	case __VCPU_XREG_R28: asm(".byte 0xd5, 0x4c, 0x89, 0xe0" : "=a"(*data)); break;
+	case __VCPU_XREG_R29: asm(".byte 0xd5, 0x4c, 0x89, 0xe8" : "=a"(*data)); break;
+	case __VCPU_XREG_R30: asm(".byte 0xd5, 0x4c, 0x89, 0xf0" : "=a"(*data)); break;
+	case __VCPU_XREG_R31: asm(".byte 0xd5, 0x4c, 0x89, 0xf8" : "=a"(*data)); break;
+	default: BUG();
+	}
+}
+
+static void _kvm_write_egpr(int reg, unsigned long *data)
+{
+	/* mov %rax, %r16...%r31*/
+	switch (reg) {
+	case __VCPU_XREG_R16: asm(".byte 0xd5, 0x18, 0x89, 0xc0" : : "a"(*data)); break;
+	case __VCPU_XREG_R17: asm(".byte 0xd5, 0x18, 0x89, 0xc1" : : "a"(*data)); break;
+	case __VCPU_XREG_R18: asm(".byte 0xd5, 0x18, 0x89, 0xc2" : : "a"(*data)); break;
+	case __VCPU_XREG_R19: asm(".byte 0xd5, 0x18, 0x89, 0xc3" : : "a"(*data)); break;
+	case __VCPU_XREG_R20: asm(".byte 0xd5, 0x18, 0x89, 0xc4" : : "a"(*data)); break;
+	case __VCPU_XREG_R21: asm(".byte 0xd5, 0x18, 0x89, 0xc5" : : "a"(*data)); break;
+	case __VCPU_XREG_R22: asm(".byte 0xd5, 0x18, 0x89, 0xc6" : : "a"(*data)); break;
+	case __VCPU_XREG_R23: asm(".byte 0xd5, 0x18, 0x89, 0xc7" : : "a"(*data)); break;
+	case __VCPU_XREG_R24: asm(".byte 0xd5, 0x19, 0x89, 0xc0" : : "a"(*data)); break;
+	case __VCPU_XREG_R25: asm(".byte 0xd5, 0x19, 0x89, 0xc1" : : "a"(*data)); break;
+	case __VCPU_XREG_R26: asm(".byte 0xd5, 0x19, 0x89, 0xc2" : : "a"(*data)); break;
+	case __VCPU_XREG_R27: asm(".byte 0xd5, 0x19, 0x89, 0xc3" : : "a"(*data)); break;
+	case __VCPU_XREG_R28: asm(".byte 0xd5, 0x19, 0x89, 0xc4" : : "a"(*data)); break;
+	case __VCPU_XREG_R29: asm(".byte 0xd5, 0x19, 0x89, 0xc5" : : "a"(*data)); break;
+	case __VCPU_XREG_R30: asm(".byte 0xd5, 0x19, 0x89, 0xc6" : : "a"(*data)); break;
+	case __VCPU_XREG_R31: asm(".byte 0xd5, 0x19, 0x89, 0xc7" : : "a"(*data)); break;
+	default: BUG();
+	}
+}
+
 static unsigned long kvm_read_egpr(int reg)
 {
-	return 0;
+	unsigned long data;
+
+	if (WARN_ON_ONCE(!cpu_has_xfeatures(XFEATURE_MASK_APX, NULL)))
+		return 0;
+
+	kvm_fpu_get();
+	_kvm_read_egpr(reg, &data);
+	kvm_fpu_put();
+
+	return data;
 }
 
 static void kvm_write_egpr(int reg, unsigned long data)
 {
+	if (WARN_ON_ONCE(!cpu_has_xfeatures(XFEATURE_MASK_APX, NULL)))
+		return;
+
+	kvm_fpu_get();
+	_kvm_write_egpr(reg, &data);
+	kvm_fpu_put();
 }
 
 unsigned long kvm_gpr_read_raw(struct kvm_vcpu *vcpu, int reg)
-- 
2.51.0