linux-kernel - Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter

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Message-ID: <YEnX3Zn0FXPt7pcM@hirez.programming.kicks-ass.net>
Date:   Thu, 11 Mar 2021 09:42:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v22 8/8] x86/vdso: Add ENDBR64 to __vdso_sgx_enter_enclave

On Thu, Mar 11, 2021 at 05:36:06AM +0200, Jarkko Sakkinen wrote:
> Does it do any harm to put it there unconditionally?

Blows up your text footprint and I$ pressure. These instructions are 4
bytes each.

Aside from that, they're a NOP, so only consume front-end resources
(hopefully) on older CPUs and when IBT is disabled.