linux-kernel - [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu

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Date:   Thu,  9 Sep 2021 15:08:16 -0700
From:   "H. Peter Anvin (Intel)" <hpa@...or.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        x86@...nel.org, "H. Peter Anvin (Intel)" <hpa@...or.com>
Subject: [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has()

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