linux-kernel - [PATCH v3 2/2] x86/asm: avoid adding register pressure for the init case in static_cpu

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Date:   Fri, 10 Sep 2021 12:59:10 -0700
From:   "H. Peter Anvin (Intel)" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     x86 mailing list <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v3 2/2] x86/asm: avoid adding register pressure for the init case in static_cpu_has()

From: "H. Peter Anvin" <hpa@...or.com>

gcc will sometimes manifest the address of boot_cpu_data in a register
as part of constant propagation. When multiple static_cpu_has() are
used this may foul the mainline code with a register load which will
only be used on the fallback path, which is unused after
initialization.

Explicitly force gcc to use immediate (rip-relative) addressing for
the fallback path, thus removing any possible register use from
static_cpu_has().

While making changes, modernize the code to use
.pushsection...popsection instead of .section...previous.

Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
---
 arch/x86/include/asm/cpufeature.h | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 16a51e7288d5..1261842d006c 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -173,20 +173,25 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
  * means that the boot_cpu_has() variant is already fast enough for the
  * majority of cases and you should stick to using it as it is generally
  * only two instructions: a RIP-relative MOV and a TEST.
+ *
+ * Do not use an "m" constraint for [cap_byte] here: gcc doesn't know
+ * that this is only used on a fallback path and will sometimes cause
+ * it to manifest the address of boot_cpu_data in a register, fouling
+ * the mainline (post-initialization) code.
  */
 static __always_inline bool _static_cpu_has(u16 bit)
 {
 	asm_volatile_goto(
 		ALTERNATIVE_TERNARY("jmp 6f", %P[feature], "", "jmp %l[t_no]")
-		".section .altinstr_aux,\"ax\"\n"
+		".pushsection .altinstr_aux,\"ax\"\n"
 		"6:\n"
-		" testb %[bitnum],%[cap_byte]\n"
+		" testb %[bitnum]," _ASM_RIP(%P[cap_byte]) "\n"
 		" jnz %l[t_yes]\n"
 		" jmp %l[t_no]\n"
-		".previous\n"
+		".popsection\n"
 		 : : [feature]  "i" (bit),
 		     [bitnum]   "i" (1 << (bit & 7)),
-		     [cap_byte] "m" (((const char *)boot_cpu_data.x86_capability)[bit >> 3])
+		     [cap_byte] "i" (&((const char *)boot_cpu_data.x86_capability)[bit >> 3])
 		 : : t_yes, t_no);
 t_yes:
 	return true;
-- 
2.31.1