[tip: x86/mm] x86/mm: Use %RIP-relative address in untagged_addr()

[tip-bot2 for Uros Bizjak]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     bc5607d7777423b742f5b0f7a760d074154c613f
Gitweb:        https://git.kernel.org/tip/bc5607d7777423b742f5b0f7a760d074154c613f
Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate:    Thu, 16 Nov 2023 20:10:59 +01:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Fri, 17 Nov 2023 06:27:29 -08:00

x86/mm: Use %RIP-relative address in untagged_addr()

%RIP-relative addresses are nowadays correctly handled in alternative
instructions, so remove misleading comment and improve assembly to
use %RIP-relative address.

Also, explicitly using %gs: prefix will segfault for non-SMP builds.
Use macros from percpu.h which will DTRT with segment prefix register
as far as SMP/non-SMP builds are concerned.

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20231116191127.3446476-1-ubizjak%40gmail.com
---
 arch/x86/include/asm/uaccess_64.h | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index f2c02e4..01455c0 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -11,6 +11,7 @@
 #include <asm/alternative.h>
 #include <asm/cpufeatures.h>
 #include <asm/page.h>
+#include <asm/percpu.h>
 
 #ifdef CONFIG_ADDRESS_MASKING
 /*
@@ -18,14 +19,10 @@
  */
 static inline unsigned long __untagged_addr(unsigned long addr)
 {
-	/*
-	 * Refer tlbstate_untag_mask directly to avoid RIP-relative relocation
-	 * in alternative instructions. The relocation gets wrong when gets
-	 * copied to the target place.
-	 */
 	asm (ALTERNATIVE("",
-			 "and %%gs:tlbstate_untag_mask, %[addr]\n\t", X86_FEATURE_LAM)
-	     : [addr] "+r" (addr) : "m" (tlbstate_untag_mask));
+			 "and " __percpu_arg([mask]) ", %[addr]", X86_FEATURE_LAM)
+	     : [addr] "+r" (addr)
+	     : [mask] "m" (__my_cpu_var(tlbstate_untag_mask)));
 
 	return addr;
 }