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

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Date:   Wed, 25 Oct 2023 08:06:44 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, tony.luck@...el.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-doc@...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,
        Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH  v2 1/6] x86/bugs: Add asm helpers for executing VERW

On Wed, Oct 25, 2023 at 08:56:10AM +0200, Peter Zijlstra wrote:
> > config3: 32-bit mode, pre-boot objdump
> > 
> > entry_SYSENTER_32:
> >    ...
> >    c8e:       58                      pop    %eax
> >    c8f:       90                      nop
> >    c90:       90                      nop
> >    c91:       90                      nop
> >    c92:       90                      nop
> >    c93:       90                      nop
> >    c94:       90                      nop
> >    c95:       90                      nop
> >    c96:       fb                      sti
> >    c97:       0f 35                   sysexit
> > 
> 
> If you look at arch/x86/include/asm/nops.h, you'll find (for 32bit):
> 
>  * 7: leal 0x0(%esi,%eiz,1),%esi
> 
> Which reads as:
> 
> 	load-effective-address of %esi[0] into %esi

Wow, never imagined that this would be one of the magician's trick. I
will go read on why is it better than NOPL.