linux-kernel - Re: [PATCH v2 1/6] x86/bugs: Add asm helpers for executing VERW

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Message-ID: <20231024223023.GI33965@noisy.programming.kicks-ass.net>
Date:   Wed, 25 Oct 2023 00:30:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        "antonio.gomez.iglesias@...ux.intel.com" 
        <antonio.gomez.iglesias@...ux.intel.com>,
        "Milburn, Alyssa" <alyssa.milburn@...el.com>
Subject: Re: [PATCH  v2 1/6] x86/bugs: Add asm helpers for executing VERW

On Tue, Oct 24, 2023 at 06:49:07PM +0000, Luck, Tony wrote:
> > the only overhead to modules other than load time (including the runtime linking) is that modules can't realistically be mapped using large page entries.
> 
> If there were some significant win for using large pages, couldn't the

There is. The 4k TLBs really hurt. Thomas and me ran into that when
doing the retbleed call depth crud. Similarly Song ran into it with BPF,
they really want eBPF JIT to write to large pages.