linux-kernel - Re: [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry trampoline

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Date:   Thu, 30 Nov 2017 21:56:41 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3 14/19] x86/entry/64: Create a percpu SYSCALL entry trampoline

On Fri, Nov 24, 2017 at 3:39 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Andy Lutomirski
>> Sent: 24 November 2017 04:33
> ...
>> +     /*
>> +      * x86 lacks a near absolute jump, and we can't jump to the real
>> +      * entry text with a relative jump, so we fake it using retq.
>> +      */
>> +     pushq   $entry_SYSCALL_64_after_hwframe
>> +     retq
>
> Don't some of the cpus have hardware logic that follows call and return?
> The above will break that logic and slow things down (unless it is an
> expected sequence).

Indeed, and it makes a *huge* difference.

>
> Is there a scratch register that can be used for an indirect jump?

No, but I can fudge it.

>
>         David
>