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From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
To: Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@kernel.org>
Cc: x86 mailing list <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: [PATCH v3 2/2] x86/asm: avoid adding register pressure for the init case in static_cpu_has()
Date: Fri, 10 Sep 2021 12:59:10 -0700	[thread overview]
Message-ID: <20210910195910.2542662-4-hpa@zytor.com> (raw)
In-Reply-To: <20210910195910.2542662-1-hpa@zytor.com>

From: "H. Peter Anvin" <hpa@zytor.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@zytor.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


  parent reply	other threads:[~2021-09-10 19:59 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 17:17 [PATCH] x86/asm: pessimize the pre-initialization " H. Peter Anvin (Intel)
2021-09-09 17:01 ` Borislav Petkov
2021-09-09 21:28   ` H. Peter Anvin
2021-09-09 21:53     ` Borislav Petkov
2021-09-09 22:17     ` H. Peter Anvin
2021-09-10  9:14       ` Borislav Petkov
2021-09-10 19:25         ` H. Peter Anvin
2021-09-09 22:08 ` [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has() H. Peter Anvin (Intel)
2021-09-09 22:08   ` [PATCH v2 1/2] x86/asm: add _ASM_RIP() macro for x86-64 (%rip) suffix H. Peter Anvin (Intel)
2021-09-09 22:08   ` [PATCH v2 2/2] x86/asm: pessimize the pre-initialization case in static_cpu_has() H. Peter Anvin (Intel)
2021-09-10  9:16   ` [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has() Borislav Petkov
2021-09-10 13:24     ` Borislav Petkov
2021-09-10 19:59 ` [PATCH v3 0/2] x86/asm: avoid register pressure from the init case in static_cpu_has() H. Peter Anvin (Intel)
2021-09-10 19:59   ` [PATCH] drm/bochs: add Bochs PCI ID for Simics model H. Peter Anvin (Intel)
2021-09-10 19:59   ` [PATCH v3 1/2] x86/asm: add _ASM_RIP() macro for x86-64 (%rip) suffix H. Peter Anvin (Intel)
2021-09-13 19:39     ` [tip: x86/cpu] x86/asm: Add " tip-bot2 for H. Peter Anvin (Intel)
2021-09-10 19:59   ` H. Peter Anvin (Intel) [this message]
2021-09-13 19:39     ` [tip: x86/cpu] x86/asm: Avoid adding register pressure for the init case in static_cpu_has() tip-bot2 for H. Peter Anvin

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