linux-kernel - Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

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Date:   Tue, 17 Oct 2017 10:05:47 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Mike Galbraith <efault@....de>, xen-devel@...ts.xenproject.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sasha Levin <alexander.levin@...izon.com>,
        Chris Wright <chrisw@...s-sol.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        live-patching@...r.kernel.org, Alok Kataria <akataria@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives
 infrastructure

On 10/17/2017 09:10 AM, Brian Gerst wrote:
> On Mon, Oct 16, 2017 at 2:18 PM, Boris Ostrovsky
> <boris.ostrovsky@...cle.com> wrote:
>>
>> Replacing the macro with
>>
>> #define PV_INDIRECT(addr)       *addr  // well, it's not so much
>> indirect anymore
>>
>> makes things work. Or maybe it can be adjusted top be kept truly indirect.
> That is still an indirect call, just using absolute addressing for the
> pointer instead of RIP-relative. 

Oh, right, I've got my terminology all wrong.

-boris

>  Alternatives has very limited
> relocation capabilities.  It will only handle a single call or jmp
> replacement. Using absolute addressing is slightly less efficient
> (takes one extra byte to encode, and needs a relocation for KASLR),
> but it works just as well.  You could also relocate the instruction
> manually by adding the delta between the original and replacement code
> to the displacement.