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

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Message-ID: <SJ1PR11MB6083E3E2D35B30F4E40E8FE7FCDFA@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date:   Tue, 24 Oct 2023 18:49:07 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
CC:     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

> 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
kernel pre-allocate some 2MB pages in the [-2GiB,0) range?  Boot parameter
for how many (perhaps two for separate code/data pages). First few loaded
modules get to use that space until it is all gone.

It would all be quite messy if those modules were later unloaded/reloaded
... so there would have to be some compelling benchmarks to justify
the complexity.

That's probably why Peter said "can't realistically".

-Tony