KVM: x86: use a separate asm-offsets.c file

This already removes an ugly #include "" from asm-offsets.c, but
especially it avoids a future error when trying to define asm-offsets
for KVM's svm/svm.h header.

This would not work for kernel/asm-offsets.c, because svm/svm.h
includes kvm_cache_regs.h which is not in the include path when
compiling asm-offsets.c.  The problem is not there if the .c file is
in arch/x86/kvm.
Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: stable@vger.kernel.org
Fixes: a149180f ("x86: Add magic AMD return-thunk")
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/asm-offsets.c

@@ -19,7 +19,6 @@
 #include <asm/suspend.h>
 #include <asm/tlbflush.h>
 #include <asm/tdx.h>
-#include "../kvm/vmx/vmx.h"
 
 #ifdef CONFIG_XEN
 #include <xen/interface/xen.h>
@@ -108,9 +107,4 @@ static void __used common(void)
 	OFFSET(TSS_sp0, tss_struct, x86_tss.sp0);
 	OFFSET(TSS_sp1, tss_struct, x86_tss.sp1);
 	OFFSET(TSS_sp2, tss_struct, x86_tss.sp2);
-
-	if (IS_ENABLED(CONFIG_KVM_INTEL)) {
-		BLANK();
-		OFFSET(VMX_spec_ctrl, vcpu_vmx, spec_ctrl);
-	}
 }

arch/x86/kvm/.gitignore

+/kvm-asm-offsets.s
+/kvm-asm-offsets.h

arch/x86/kvm/Makefile

@@ -34,3 +34,12 @@ endif
 obj-$(CONFIG_KVM)	+= kvm.o
 obj-$(CONFIG_KVM_INTEL)	+= kvm-intel.o
 obj-$(CONFIG_KVM_AMD)	+= kvm-amd.o
+
+AFLAGS_vmx/vmenter.o    := -iquote $(obj)
+$(obj)/vmx/vmenter.o: $(obj)/kvm-asm-offsets.h
+
+$(obj)/kvm-asm-offsets.h: $(obj)/kvm-asm-offsets.s FORCE
+	$(call filechk,offsets,__KVM_ASM_OFFSETS_H__)
+
+targets += kvm-asm-offsets.s
+clean-files += kvm-asm-offsets.h

arch/x86/kvm/kvm-asm-offsets.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generate definitions needed by assembly language modules.
+ * This code generates raw asm output which is post-processed to extract
+ * and format the required data.
+ */
+#define COMPILE_OFFSETS
+
+#include <linux/kbuild.h>
+#include "vmx/vmx.h"
+
+static void __used common(void)
+{
+	if (IS_ENABLED(CONFIG_KVM_INTEL)) {
+		BLANK();
+		OFFSET(VMX_spec_ctrl, vcpu_vmx, spec_ctrl);
+	}
+}

arch/x86/kvm/vmx/vmenter.S

 /* SPDX-License-Identifier: GPL-2.0 */
 #include <linux/linkage.h>
 #include <asm/asm.h>
-#include <asm/asm-offsets.h>
 #include <asm/bitsperlong.h>
 #include <asm/kvm_vcpu_regs.h>
 #include <asm/nospec-branch.h>
 #include <asm/percpu.h>
 #include <asm/segment.h>
+#include "kvm-asm-offsets.h"
 #include "run_flags.h"
 
 #define WORD_SIZE (BITS_PER_LONG / 8)