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

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Message-ID: <5B8EB5F2-16A7-47BC-97FE-262ED0169DE3@zytor.com>
Date:   Tue, 24 Oct 2023 12:14:25 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Luck, Tony" <tony.luck@...el.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

On October 24, 2023 11:49:07 AM PDT, "Luck, Tony" <tony.luck@...el.com> 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
>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
>

Sure it could, but it would mean the kernel is sitting on an average of 6 MB of unusable memory. It would also mean that unloaded modules would create holes in that memory which would have to be managed.