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

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Date:   Wed, 25 Oct 2023 10:02:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        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: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

On Wed, Oct 25, 2023 at 08:52:50AM +0100, Andrew Cooper wrote:

> > +.pushsection .entry.text, "ax"
> > +
> > +.align 64
> > +SYM_CODE_START_NOALIGN(mds_verw_sel)
> > +	UNWIND_HINT_UNDEFINED
> > +	ANNOTATE_NOENDBR
> > +1:
> > +	.word __KERNEL_DS
> > +	.skip 64 - (. - 1b), 0xcc
> 
> The 1 label aliases mds_verw_sel and this must remain like this for the
> construct to work.
> 
> So instead of .skip, why not simply .align 64, 0xcc and get rid of the
> 1: label?

Because I forgot you can add a filler byte to .align :/ Yes, that's much
saner.

> Do we have a suitably named constant cacheline size, rather than
> opencoding 64?

L1_CACHE_BYTES probably.

> 
> > +SYM_CODE_END(mds_verw_sel);
> 
> Given that KVM needs it, this probably needs an EXPORT_SYMBOL_GPL() on it.

localyesconfig ftw ;-)

/me runs