From: Borislav Petkov <bp@alien8.de>
To: "H. Peter Anvin (Intel)" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
x86@kernel.org
Subject: Re: [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has()
Date: Fri, 10 Sep 2021 11:16:47 +0200 [thread overview]
Message-ID: <YTsif1FeOEyFJnqc@zn.tnic> (raw)
In-Reply-To: <20210909220818.417312-1-hpa@zytor.com>
On Thu, Sep 09, 2021 at 03:08:16PM -0700, H. Peter Anvin (Intel) wrote:
> 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().
Right, maybe I'm missing something but what is wrong with the immediate
addressing variant, i.e., that thing:
testb $0x8,0xffffffff89346eea
and you need to *force* %rip-relative?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2021-09-10 9:17 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 case in static_cpu_has() 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 ` Borislav Petkov [this message]
2021-09-10 13:24 ` [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has() 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 ` [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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