[PATCH v2 2/7] KVM: VMX: more cleanups to __vmx_vcpu_run

Paolo Bonzini
2026-04-27T10:58:56Z

Slightly improve register allocation, loading vmx only once
before vmlaunch/vmresume.

This also makes the code slightly more similar to the one for
AMD processors, in that both keep the pointer to struct vcpu_vmx
or vcpu_svm in %rdi. The code for restoring the guest value of
SPEC_CTRL is also the same for Intel and AMD.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/vmx/vmenter.S | 85 ++++++++++++++++++--------------------
1 file changed, 41 insertions(+), 44 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 0a09288a8d29..efac31cedfde 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -98,11 +98,11 @@ SYM_FUNC_START(__vmx_vcpu_run)
lea (%_ASM_SP), %_ASM_ARG2
call vmx_update_host_rsp

- ALTERNATIVE "jmp .Lspec_ctrl_done", "", X86_FEATURE_MSR_SPEC_CTRL
-
/* Reload @vmx, _ASM_ARG1 may be modified by vmx_update_host_rsp(). */
mov WORD_SIZE(%_ASM_SP), %_ASM_DI

+ ALTERNATIVE "jmp .Lspec_ctrl_done", "", X86_FEATURE_MSR_SPEC_CTRL
+
/*
* SPEC_CTRL handling: if the guest's SPEC_CTRL value differs from the
* host's, write the MSR.
@@ -122,9 +122,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
mov PER_CPU_VAR(x86_spec_ctrl_current), %ecx
xor %eax, %ecx
mov VMX_spec_ctrl + 4(%edi), %edx
- mov PER_CPU_VAR(x86_spec_ctrl_current + 4), %edi
- xor %edx, %edi
- or %edi, %ecx
+ mov PER_CPU_VAR(x86_spec_ctrl_current + 4), %esi
+ xor %edx, %esi
+ or %esi, %ecx
je .Lspec_ctrl_done
#endif
mov $MSR_IA32_SPEC_CTRL, %ecx
@@ -137,28 +137,25 @@ SYM_FUNC_START(__vmx_vcpu_run)
* an LFENCE to stop speculation from skipping the wrmsr.
*/

- /* Load @vmx to RAX. */
- mov WORD_SIZE(%_ASM_SP), %_ASM_AX
-
/* Load guest registers. Don't clobber flags. */
- mov VCPU_RCX(%_ASM_AX), %_ASM_CX
- mov VCPU_RDX(%_ASM_AX), %_ASM_DX
- mov VCPU_RBX(%_ASM_AX), %_ASM_BX
- mov VCPU_RBP(%_ASM_AX), %_ASM_BP
- mov VCPU_RSI(%_ASM_AX), %_ASM_SI
- mov VCPU_RDI(%_ASM_AX), %_ASM_DI
+ mov VCPU_RAX(%_ASM_DI), %_ASM_AX
+ mov VCPU_RCX(%_ASM_DI), %_ASM_CX
+ mov VCPU_RDX(%_ASM_DI), %_ASM_DX
+ mov VCPU_RBX(%_ASM_DI), %_ASM_BX
+ mov VCPU_RBP(%_ASM_DI), %_ASM_BP
+ mov VCPU_RSI(%_ASM_DI), %_ASM_SI
#ifdef CONFIG_X86_64
- mov VCPU_R8 (%_ASM_AX), %r8
- mov VCPU_R9 (%_ASM_AX), %r9
- mov VCPU_R10(%_ASM_AX), %r10
- mov VCPU_R11(%_ASM_AX), %r11
- mov VCPU_R12(%_ASM_AX), %r12
- mov VCPU_R13(%_ASM_AX), %r13
- mov VCPU_R14(%_ASM_AX), %r14
- mov VCPU_R15(%_ASM_AX), %r15
+ mov VCPU_R8 (%_ASM_DI), %r8
+ mov VCPU_R9 (%_ASM_DI), %r9
+ mov VCPU_R10(%_ASM_DI), %r10
+ mov VCPU_R11(%_ASM_DI), %r11
+ mov VCPU_R12(%_ASM_DI), %r12
+ mov VCPU_R13(%_ASM_DI), %r13
+ mov VCPU_R14(%_ASM_DI), %r14
+ mov VCPU_R15(%_ASM_DI), %r15
#endif
- /* Load guest RAX. This kills the @vmx pointer! */
- mov VCPU_RAX(%_ASM_AX), %_ASM_AX
+ /* Load guest RDI. This kills the @vmx pointer! */
+ mov VCPU_RDI(%_ASM_DI), %_ASM_DI

/*
* Note, ALTERNATIVE_2 works in reverse order. If CLEAR_CPU_BUF_VM is
@@ -207,29 +204,29 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
UNWIND_HINT_RESTORE
ENDBR

- /* Temporarily save guest's RAX. */
- push %_ASM_AX
+ /* Temporarily save guest's RDI. */
+ push %_ASM_DI

- /* Reload @vmx to RAX. */
- mov 2*WORD_SIZE(%_ASM_SP), %_ASM_AX
+ /* Reload @vmx to RDI. */
+ mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI

- /* Save all guest registers, including RAX from the stack */
- pop VCPU_RAX(%_ASM_AX)
- mov %_ASM_CX, VCPU_RCX(%_ASM_AX)
- mov %_ASM_DX, VCPU_RDX(%_ASM_AX)
- mov %_ASM_BX, VCPU_RBX(%_ASM_AX)
- mov %_ASM_BP, VCPU_RBP(%_ASM_AX)
- mov %_ASM_SI, VCPU_RSI(%_ASM_AX)
- mov %_ASM_DI, VCPU_RDI(%_ASM_AX)
+ /* Save all guest registers, including RDI from the stack */
+ mov %_ASM_AX, VCPU_RAX(%_ASM_DI)
+ mov %_ASM_CX, VCPU_RCX(%_ASM_DI)
+ mov %_ASM_DX, VCPU_RDX(%_ASM_DI)
+ mov %_ASM_BX, VCPU_RBX(%_ASM_DI)
+ mov %_ASM_BP, VCPU_RBP(%_ASM_DI)
+ mov %_ASM_SI, VCPU_RSI(%_ASM_DI)
+ pop VCPU_RDI(%_ASM_DI)
#ifdef CONFIG_X86_64
- mov %r8, VCPU_R8 (%_ASM_AX)
- mov %r9, VCPU_R9 (%_ASM_AX)
- mov %r10, VCPU_R10(%_ASM_AX)
- mov %r11, VCPU_R11(%_ASM_AX)
- mov %r12, VCPU_R12(%_ASM_AX)
- mov %r13, VCPU_R13(%_ASM_AX)
- mov %r14, VCPU_R14(%_ASM_AX)
- mov %r15, VCPU_R15(%_ASM_AX)
+ mov %r8, VCPU_R8 (%_ASM_DI)
+ mov %r9, VCPU_R9 (%_ASM_DI)
+ mov %r10, VCPU_R10(%_ASM_DI)
+ mov %r11, VCPU_R11(%_ASM_DI)
+ mov %r12, VCPU_R12(%_ASM_DI)
+ mov %r13, VCPU_R13(%_ASM_DI)
+ mov %r14, VCPU_R14(%_ASM_DI)
+ mov %r15, VCPU_R15(%_ASM_DI)
#endif

/* Clear return value to indicate VM-Exit (as opposed to VM-Fail). */
--
2.52.0