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

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Date:   Tue, 24 Oct 2023 18:36:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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,
        "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 Tue, Oct 24, 2023 at 09:35:15AM -0700, Pawan Gupta wrote:
> On Tue, Oct 24, 2023 at 12:36:01PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 24, 2023 at 01:08:21AM -0700, Pawan Gupta wrote:
> > 
> > > +.macro CLEAR_CPU_BUFFERS
> > > +	ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@", X86_FEATURE_CLEAR_CPU_BUF
> > > +		/* nopl __KERNEL_DS(%rax) */
> > > +		.byte 0x0f, 0x1f, 0x80, 0x00, 0x00;
> > > +.Lverw_arg_\@:	.word __KERNEL_DS;
> > > +.Ldo_verw_\@:	verw _ASM_RIP(.Lverw_arg_\@);
> > > +.Lskip_verw_\@:
> > > +.endm
> > 
> > Why can't this be:
> > 
> > 	ALTERNATIVE "". "verw _ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF
> > 
> > And have that mds_verw_sel thing be out-of-line ?
> 
> I haven't done this way because its a tad bit fragile as it depends on
> modules being within 4GB of kernel.

We 100% rely on that *everywhere*, nothing fragile about it.