[PATCH 7/7] um: Centralize stub size calculations

[Tiwei Bie <tiwei.bie@linux.dev>]

From: Tiwei Bie <tiwei.btw@antgroup.com>

Currently, the stub size is calculated in multiple places. Define
a macro that performs the calculation so that the code is easier
to read and maintain.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
---
 arch/um/include/shared/as-layout.h  | 3 ++-
 arch/um/kernel/um_arch.c            | 4 +---
 arch/x86/um/shared/sysdep/stub_32.h | 2 +-
 arch/x86/um/shared/sysdep/stub_64.h | 2 +-
 4 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/um/include/shared/as-layout.h b/arch/um/include/shared/as-layout.h
index eacf18ec9d0c..7c7e17bce403 100644
--- a/arch/um/include/shared/as-layout.h
+++ b/arch/um/include/shared/as-layout.h
@@ -24,7 +24,8 @@
 #define STUB_CODE STUB_START
 #define STUB_DATA (STUB_CODE + UM_KERN_PAGE_SIZE)
 #define STUB_DATA_PAGES 2
-#define STUB_END (STUB_DATA + STUB_DATA_PAGES * UM_KERN_PAGE_SIZE)
+#define STUB_SIZE ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE)
+#define STUB_END (STUB_START + STUB_SIZE)
 
 #ifndef __ASSEMBLER__
 
diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index 2f5ee045bc7a..6cd464c9095f 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -331,9 +331,7 @@ int __init linux_main(int argc, char **argv, char **envp)
 
 	host_task_size = get_top_address(envp);
 	/* reserve a few pages for the stubs */
-	stub_start = host_task_size - STUB_DATA_PAGES * PAGE_SIZE;
-	/* another page for the code portion */
-	stub_start -= PAGE_SIZE;
+	stub_start = host_task_size - STUB_SIZE;
 	host_task_size = stub_start;
 
 	/* Limit TASK_SIZE to what is addressable by the page table */
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index df568fc3ceb4..9dc2efaf5df1 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -129,7 +129,7 @@ static __always_inline void *get_stub_data(void)
 		"subl %0,%%esp ;"					\
 		"movl %1, %%eax ; "					\
 		"call *%%eax ;"						\
-		:: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE),	\
+		:: "i" (STUB_SIZE),					\
 		   "i" (&fn))
 
 static __always_inline void
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 9cfd31afa769..9fd56954e2e0 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -133,7 +133,7 @@ static __always_inline void *get_stub_data(void)
 		"subq %0,%%rsp ;"					\
 		"movq %1,%%rax ;"					\
 		"call *%%rax ;"						\
-		:: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE),	\
+		:: "i" (STUB_SIZE),					\
 		   "i" (&fn))
 
 static __always_inline void
-- 
2.34.1