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From:   "H. Peter Anvin (Intel)" <hpa@zytor.com>
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        x86@kernel.org, "H. Peter Anvin (Intel)" <hpa@zytor.com>
Subject: [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu_has()
Date:   Thu,  9 Sep 2021 15:08:16 -0700
Message-Id: <20210909220818.417312-1-hpa@zytor.com>
In-Reply-To: <20210908171716.3340120-1-hpa@zytor.com>
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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.