linux-kernel - Re: [RFC 0/3] x86: Patchable constants

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Message-ID: <83ddad10-936f-9848-e3d1-8c6e73ab2969@zytor.com>
Date:   Wed, 7 Feb 2018 12:43:33 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>, linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 0/3] x86: Patchable constants

FYI: the interface I'm implementing looks like this:

	[output =] const_op(data, variable);

... where variable can be any variable with a static address, although
in any sane scenario it should be __ro_after_init.  During
initialization, it kicks out to an out-of-line (.alternatives_aux)
handler which accesses the variable in normal fashion, but at
alternatives-patching time it inlines the relevant opcodes.

Some of the assembly infrastructure for this is indeed hairy, especially
when the out-of-line hander needs temp registers.

	-hpa