linux-kernel - [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

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Message-ID: <20260120195407.1163051-13-hpa@zytor.com>
Date: Tue, 20 Jan 2026 11:54:04 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Uros Bizjak <ubizjak@...il.com>, Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Kiryl Shutsemau <kas@...nel.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-coco@...ts.linux.dev, x86@...nel.org
Subject: [PATCH v1 12/14] x86/boot: tweak a20.c for better code generation

Do some minor tweaks to arch/x86/boot/a20.c for better code
generation, made possible by the __seg_fs/__seg_gs changes.

Move the die() call to a20.c itself; there is no reason to push an
error code upwards just to die() there.

Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
---
 arch/x86/boot/a20.c  | 24 +++++++++++-------------
 arch/x86/boot/boot.h |  2 +-
 arch/x86/boot/pm.c   |  3 +--
 3 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
index 52c3fccdcb70..38a1cad8a553 100644
--- a/arch/x86/boot/a20.c
+++ b/arch/x86/boot/a20.c
@@ -53,24 +53,22 @@ static int empty_8042(void)
 
 static int a20_test(int loops)
 {
-	int ok = 0;
 	int saved, ctr;
 
 	set_fs(0xffff);
 
 	saved = ctr = rdgs32(A20_TEST_ADDR);
 
-	while (loops--) {
+	do {
 		wrgs32(++ctr, A20_TEST_ADDR);
 		io_delay();	/* Serialize and make delay constant */
 		barrier();	/* Compiler won't know about fs/gs overlap */
-		ok = rdfs32(A20_TEST_ADDR+0x10) ^ ctr;
-		if (ok)
+		if (rdfs32(A20_TEST_ADDR+0x10) != ctr)
 			break;
-	}
+	} while (--loops);
 
 	wrgs32(saved, A20_TEST_ADDR);
-	return ok;
+	return loops;
 }
 
 /* Quick test to see if A20 is already enabled */
@@ -125,7 +123,7 @@ static void enable_a20_fast(void)
 
 #define A20_ENABLE_LOOPS 255	/* Number of times to try */
 
-int enable_a20(void)
+void enable_a20(void)
 {
        int loops = A20_ENABLE_LOOPS;
        int kbc_err;
@@ -134,30 +132,30 @@ int enable_a20(void)
 	       /* First, check to see if A20 is already enabled
 		  (legacy free, etc.) */
 	       if (a20_test_short())
-		       return 0;
+		       return;
 
 	       /* Next, try the BIOS (INT 0x15, AX=0x2401) */
 	       enable_a20_bios();
 	       if (a20_test_short())
-		       return 0;
+		       return;
 
 	       /* Try enabling A20 through the keyboard controller */
 	       kbc_err = empty_8042();
 
 	       if (a20_test_short())
-		       return 0; /* BIOS worked, but with delayed reaction */
+		       return; /* BIOS worked, but with delayed reaction */
 
 	       if (!kbc_err) {
 		       enable_a20_kbc();
 		       if (a20_test_long())
-			       return 0;
+			      return;
 	       }
 
 	       /* Finally, try enabling the "fast A20 gate" */
 	       enable_a20_fast();
 	       if (a20_test_long())
-		       return 0;
+		       return;
        }
 
-       return -1;
+       die("A20 gate not responding, unable to boot...\n");
 }
diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
index 4d3549ed7987..584c89d0738b 100644
--- a/arch/x86/boot/boot.h
+++ b/arch/x86/boot/boot.h
@@ -167,7 +167,7 @@ void copy_to_fs(addr_t dst, void *src, size_t len);
 void *copy_from_fs(void *dst, addr_t src, size_t len);
 
 /* a20.c */
-int enable_a20(void);
+void enable_a20(void);
 
 /* apm.c */
 int query_apm_bios(void);
diff --git a/arch/x86/boot/pm.c b/arch/x86/boot/pm.c
index 3be89ba4b1b3..e39689ed65ea 100644
--- a/arch/x86/boot/pm.c
+++ b/arch/x86/boot/pm.c
@@ -106,8 +106,7 @@ void go_to_protected_mode(void)
 	realmode_switch_hook();
 
 	/* Enable the A20 gate */
-	if (enable_a20())
-		die("A20 gate not responding, unable to boot...\n");
+	enable_a20();
 
 	/* Reset coprocessor (IGNNE#) */
 	reset_coprocessor();
-- 
2.52.0