KVM: VMX: Use on-stack copy of @flags in __vmx_vcpu_run()

When testing for VMLAUNCH vs. VMRESUME, use the copy of @flags from the
stack instead of first moving it to EBX, and then propagating
VMX_RUN_VMRESUME to RFLAGS.CF (because RBX is clobbered with the guest
value prior to the conditional branch to VMLAUNCH).  Stashing information
in RFLAGS is gross, especially with the writer and reader being bifurcated
by yet more gnarly assembly code.

Opportunistically drop the SHIFT macros as they existed purely to allow
the VM-Enter flow to use Bit Test.

Suggested-by: Borislav Petkov <bp@alien8.de>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Link: https://patch.msgid.link/20251113233746.1703361-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/run_flags.h

@@ -2,12 +2,8 @@
 #ifndef __KVM_X86_VMX_RUN_FLAGS_H
 #define __KVM_X86_VMX_RUN_FLAGS_H
 
-#define VMX_RUN_VMRESUME_SHIFT				0
-#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT			1
-#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT	2
-
-#define VMX_RUN_VMRESUME			BIT(VMX_RUN_VMRESUME_SHIFT)
-#define VMX_RUN_SAVE_SPEC_CTRL			BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT)
-#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO	BIT(VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT)
+#define VMX_RUN_VMRESUME			BIT(0)
+#define VMX_RUN_SAVE_SPEC_CTRL			BIT(1)
+#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO	BIT(2)
 
 #endif /* __KVM_X86_VMX_RUN_FLAGS_H */

arch/x86/kvm/vmx/vmenter.S

@@ -92,7 +92,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Save @vmx for SPEC_CTRL handling */
 	push %_ASM_ARG1
 
-	/* Save @flags for SPEC_CTRL handling */
+	/* Save @flags (used for VMLAUNCH vs. VMRESUME and mitigations). */
 	push %_ASM_ARG3
 
 	/*
@@ -101,9 +101,6 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	 */
 	push %_ASM_ARG2
 
-	/* Copy @flags to EBX, _ASM_ARG3 is volatile. */
-	mov %_ASM_ARG3L, %ebx
-
 	lea (%_ASM_SP), %_ASM_ARG2
 	call vmx_update_host_rsp
 
@@ -137,9 +134,6 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Load @regs to RAX. */
 	mov (%_ASM_SP), %_ASM_AX
 
-	/* Check if vmlaunch or vmresume is needed */
-	bt   $VMX_RUN_VMRESUME_SHIFT, %ebx
-
 	/* Load guest registers.  Don't clobber flags. */
 	mov VCPU_RCX(%_ASM_AX), %_ASM_CX
 	mov VCPU_RDX(%_ASM_AX), %_ASM_DX
@@ -163,8 +157,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
 	/* Clobbers EFLAGS.ZF */
 	CLEAR_CPU_BUFFERS
 
-	/* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
-	jnc .Lvmlaunch
+	/* Check @flags to see if VMLAUNCH or VMRESUME is needed. */
+	testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP)
+	jz .Lvmlaunch
 
 	/*
 	 * After a successful VMRESUME/VMLAUNCH, control flow "magically"