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

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Message-ID: <SJ1PR11MB6083FE98A35C6BCF027B568CFCDFA@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date:   Tue, 24 Oct 2023 19:40:02 +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

> 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.

On my Fedora38 desktop:

$ lsmod | awk '{ bytes += $2 } END {print bytes/(1024*1024)}'
21.0859

Lots more than 6MB memory already essentially pinned by loaded modules.

$ head -3 /proc/meminfo
MemTotal:       65507344 kB
MemFree:        56762336 kB
MemAvailable:   63358552 kB

Pinning 20 or so Mbytes isn't going to make a dent in that free memory.

Managing the holes for unloading/reloading modules adds some complexity ... but shouldn't be awful.

If this code managed at finer granularity than "page", it would save some memory.

$ lsmod | wc -l
123

All those modules rounding text/data up to 4K boundaries is wasting a bunch of it.

-Tony