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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 (Intel)" <hpa@zytor.com>
Subject: [PATCH v3 0/2] x86/asm: avoid register pressure from the init case in static_cpu_has()
Date: Fri, 10 Sep 2021 12:59:07 -0700	[thread overview]
Message-ID: <20210910195910.2542662-1-hpa@zytor.com> (raw)
In-Reply-To: <20210908171716.3340120-1-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().

However, currently there is no convenient way to make gcc generate a
%rip-relative immediate reference without splitting code into i386 and
x86-64 versions, so add a new macro to <asm/asm.h> for this purpose.

Changes in v3:
--------------
* Clarify the subject line

Changes in v2:
--------------
* Add new macro to <asm/asm.h>
* *Actually* generate the %rip-relative addressing mode.

 arch/x86/include/asm/asm.h        |  5 +++++
 arch/x86/include/asm/cpufeature.h | 13 +++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

  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 ` H. Peter Anvin (Intel) [this message]
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   ` [PATCH v3 2/2] x86/asm: avoid adding register pressure for the init case in static_cpu_has() H. Peter Anvin (Intel)
2021-09-13 19:39     ` [tip: x86/cpu] x86/asm: Avoid " tip-bot2 for H. Peter Anvin

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