x86/boot: Fix latent memory corruption with early_boot_opts_t

c/s ebb26b509f "xen/x86: make VGA support selectable" added an #ifdef
CONFIG_VIDEO into the middle the backing space for early_boot_opts_t,
but didn't adjust the structure definition in cmdline.c

This only functions correctly because the affected fields are at the end
of the structure, and cmdline.c doesn't write to them in this case.

To retain the slimming effect of compiling out CONFIG_VIDEO, adjust
cmdline.c with enough #ifdef-ary to make C's idea of the structure match
the declaration in asm.  This requires adding __maybe_unused annotations
to two helper functions.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/boot/cmdline.c

@@ -40,10 +40,12 @@ typedef struct __packed {
     u8 opt_edd;
     u8 opt_edid;
     u8 padding;
+#ifdef CONFIG_VIDEO
     u16 boot_vid_mode;
     u16 vesa_width;
     u16 vesa_height;
     u16 vesa_depth;
+#endif
 } early_boot_opts_t;
 
 /*
@@ -127,7 +129,8 @@ static size_t strcspn(const char *s, const char *reject)
     return count;
 }
 
-static unsigned int strtoui(const char *s, const char *stop, const char **next)
+static unsigned int __maybe_unused strtoui(
+    const char *s, const char *stop, const char **next)
 {
     char base = 10, l;
     unsigned long long res = 0;
@@ -176,7 +179,7 @@ static int strmaxcmp(const char *cs, const char *ct, const char *_delim_chars)
     return strncmp(cs, ct, max(strcspn(cs, _delim_chars), strlen(ct)));
 }
 
-static int strsubcmp(const char *cs, const char *ct)
+static int __maybe_unused strsubcmp(const char *cs, const char *ct)
 {
     return strncmp(cs, ct, strlen(ct));
 }
@@ -241,6 +244,7 @@ static u8 edid_parse(const char *cmdline)
     return !strmaxcmp(c, "no", delim_chars);
 }
 
+#ifdef CONFIG_VIDEO
 static u16 rows2vmode(unsigned int rows)
 {
     switch ( rows )
@@ -328,6 +332,7 @@ static void vga_parse(const char *cmdline, early_boot_opts_t *ebo)
         ebo->boot_vid_mode = tmp;
     }
 }
+#endif
 
 void __stdcall cmdline_parse_early(const char *cmdline, early_boot_opts_t *ebo)
 {
@@ -338,6 +343,7 @@ void __stdcall cmdline_parse_early(const char *cmdline, early_boot_opts_t *ebo)
     ebo->opt_edd = edd_parse(cmdline);
     ebo->opt_edid = edid_parse(cmdline);
 
-    if ( IS_ENABLED(CONFIG_VIDEO) )
-        vga_parse(cmdline, ebo);
+#ifdef CONFIG_VIDEO
+    vga_parse(cmdline, ebo);
+#endif
 }

xen/arch/x86/boot/defs.h

@@ -22,6 +22,7 @@
 
 #include "../../../include/xen/stdbool.h"
 
+#define __maybe_unused	__attribute__((__unused__))
 #define __packed	__attribute__((__packed__))
 #define __stdcall	__attribute__((__stdcall__))