[PATCH v3 05/20] KVM: VMX: Save guest EGPRs in VCPU cache

Chang S. Bae
2026-04-28T05:26:20Z

Save and restore the guest EGPRs on VM exit/entry if the system supports
Advanced Performance Extensions (APX).

KVM switches XCR0 outside of the fastpath loop. As a result, there is a
window of still running with guest XCR0 where any EGPR access would
induce #UD. But fastpath handlers may access guest EGPRs, i.e. for exits
from an immediate form of WRMSR. So it is safe to handle them in
__vmx_vcpu_run().

KVM intercepts all XCR0 writes, allowing itself to track the guest XCR0
state. Pivot XCR0[APX] to conditionally handle them. This in turn makes
the VM behaves architecturally aligned. If XCR0[APX] is cleared, for
example, the EGPR state is preserved and will be restored when APX is
re-enabled.

Guard the switching path with speculation-safe control as well to avoid
any mis-speculation into EGPR accesses when APX is disabled.

Saving may be skipped on VM-Fail but leave it unconditional simply because
that's only for the slow path.

Link: https://lore.kernel.org/adPRA4ZhnvbaXSn0@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
---
V2 -> V3: New patch

Note this change depends on Paolo's SPEC_CTRL rework [1]

[1] https://lore.kernel.org/20260427105848.44865-1-pbonzini@redhat.com/
---
arch/x86/Kconfig.assembler | 5 +++++
arch/x86/kvm/Kconfig | 2 +-
arch/x86/kvm/vmenter.h | 13 +++++++++++++
arch/x86/kvm/vmx/vmenter.S | 26 ++++++++++++++++++++++++--
arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
5 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/arch/x86/Kconfig.assembler b/arch/x86/Kconfig.assembler
index b1c59fb0a4c9..3b41ec89468d 100644
--- a/arch/x86/Kconfig.assembler
+++ b/arch/x86/Kconfig.assembler
@@ -1,6 +1,11 @@
# SPDX-License-Identifier: GPL-2.0
# Copyright (C) 2020 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.

+config AS_APX
+ def_bool $(as-instr64,xor %r16$(comma)%r16)
+ help
+ Supported by binutils >= 2.42 and LLVM integrated assembler >= V18
+
config AS_WRUSS
def_bool $(as-instr64,wrussq %rax$(comma)(%rbx))
help
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index f27e3f2937f0..eb71cd7e5b37 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -100,7 +100,7 @@ config KVM_INTEL
tristate "KVM for Intel (and compatible) processors support"
depends on KVM && IA32_FEAT_CTL
select X86_FRED if X86_64
- select KVM_APX if X86_64
+ select KVM_APX if X86_64 && AS_APX
help
Provides support for KVM on processors equipped with Intel's VT
extensions, a.k.a. Virtual Machine Extensions (VMX).
diff --git a/arch/x86/kvm/vmenter.h b/arch/x86/kvm/vmenter.h
index 939d8a01b16d..7a314ee29cda 100644
--- a/arch/x86/kvm/vmenter.h
+++ b/arch/x86/kvm/vmenter.h
@@ -7,6 +7,7 @@
#define KVM_ENTER_VMRESUME BIT(0)
#define KVM_ENTER_SAVE_SPEC_CTRL BIT(1)
#define KVM_ENTER_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(2)
+#define KVM_ENTER_EGPR_SWITCH BIT(3)

#ifdef __ASSEMBLER__
.macro RESTORE_GUEST_SPEC_CTRL_BODY guest_spec_ctrl:req, label:req
@@ -114,5 +115,17 @@
.endm
#endif

+#ifdef CONFIG_KVM_APX
+.macro CLEAR_EGPRS
+ CLEAR_REGS 16d,17d,18d,19d,20d,21d,22d,23d,24d,25d,26d,27d,28d,29d,30d,31d
+.endm
+.macro VMX_LOAD_EGPRS src:req
+ VMX_LOAD_REGS \src, 16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31
+.endm
+.macro VMX_STORE_EGPRS dst:req
+ VMX_STORE_REGS \dst, 16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31
+.endm
+#endif
+
#endif /* __ASSEMBLER__ */
#endif /* __KVM_X86_ENTER_FLAGS_H */
diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index f8420b3a2741..f04efafbb7be 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -57,6 +57,7 @@
* @flags: KVM_ENTER_RUN_VMRESUME: use VMRESUME instead of VMLAUNCH
* KVM_ENTER_RUN_SAVE_SPEC_CTRL: save guest SPEC_CTRL into vmx->spec_ctrl
* KVM_ENTER_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO: vCPU can access host MMIO
+ * KVM_ENTER_RUN_EGPRS_SWITCH: load/store guest EGPRs
*
* Returns:
* 0 on VM-Exit, 1 on VM-Fail
@@ -87,6 +88,14 @@ SYM_FUNC_START(__vmx_vcpu_run)
/* Reload @vmx, _ASM_ARG1 may be modified by vmx_update_host_rsp(). */
mov WORD_SIZE(%_ASM_SP), %_ASM_DI

+#ifdef CONFIG_KVM_APX
+ ALTERNATIVE "jmp .Lload_egprs_done", "", X86_FEATURE_APX
+ testl $KVM_ENTER_EGPR_SWITCH, (%_ASM_SP)
+ jz .Lload_egprs_done
+ VMX_LOAD_EGPRS %_ASM_DI
+.Lload_egprs_done:
+#endif
+
/*
* Unlike AMD there's no V_SPEC_CTRL here, so do not leave the body
* out of line. Clobbers RAX, RCX, RDX, RSI.
@@ -229,8 +238,21 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
mov %_ASM_BX, %_ASM_AX

/* Pop our saved arguments from the stack */
- pop %_ASM_BX
- pop %_ASM_BX
+ pop %_ASM_BX /* @flags */
+ pop %_ASM_DI /* @vmx */
+
+#ifdef CONFIG_KVM_APX
+ ALTERNATIVE "jmp .Lclear_egprs_done", "", X86_FEATURE_APX
+ test $KVM_ENTER_EGPR_SWITCH, %_ASM_BX
+ jz .Lclear_egprs_done
+ /*
+ * Unlike legacy GPRs, saving could be conditional here on VM-Fail,
+ * which however isn't in fastpath. Instead, simply saving EGPRs always.
+ */
+ VMX_STORE_EGPRS %_ASM_DI
+ CLEAR_EGPRS
+.Lclear_egprs_done:
+#endif

/* ... and then the callee-save registers */
pop %_ASM_BX
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 05bb9d693aff..05e5005b1524 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -984,6 +984,19 @@ unsigned int __vmx_vcpu_enter_flags(struct vcpu_vmx *vmx)
kvm_vcpu_can_access_host_mmio(&vmx->vcpu))
flags |= KVM_ENTER_CLEAR_CPU_BUFFERS_FOR_MMIO;

+ /*
+ * KVM intercepts XSETBV and thus always tracks the guest XCR0. EGPR
+ * save/restore is gated by this flag. The resulting behavior is:
+ *
+ * - When the guest enables APX, KVM restores EGPRs (initially zeroed).
+ * - When the guest disables APX, EGPRs are preserved in the VCPU cache
+ * - When APX is re-enabled, the saved state is restored, which matches
+ * with architectural expectations.
+ */
+ if (IS_ENABLED(CONFIG_KVM_APX) && cpu_feature_enabled(X86_FEATURE_APX) &&
+ vmx->vcpu.arch.xcr0 & XFEATURE_MASK_APX)
+ flags |= KVM_ENTER_EGPR_SWITCH;
+
return flags;
}

--
2.51.0