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Date:   Wed, 25 Oct 2023 14:49:09 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, tony.luck@intel.com,
        ak@linux.intel.com, tim.c.chen@linux.intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kvm@vger.kernel.org,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Daniel Sneddon <daniel.sneddon@linux.intel.com>,
        antonio.gomez.iglesias@linux.intel.com,
        Alyssa Milburn <alyssa.milburn@intel.com>
Subject: Re: [PATCH v3 1/6] x86/bugs: Add asm helpers for executing VERW
Message-ID: <20231025214909.yxcldnporsnicrdi@treble>
References: <20231025-delay-verw-v3-0-52663677ee35@linux.intel.com>
 <20231025-delay-verw-v3-1-52663677ee35@linux.intel.com>
 <8b6d857f-cbf6-4969-8285-f90254bdafc0@citrix.com>
 <20231025212806.pgykrxzcmbhrhix5@treble>
 <da782a61-e7f6-45fa-88e9-9d974dcc1a87@citrix.com>
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On Wed, Oct 25, 2023 at 10:30:52PM +0100, Andrew Cooper wrote:
> On 25/10/2023 10:28 pm, Josh Poimboeuf wrote:
> > On Wed, Oct 25, 2023 at 10:10:41PM +0100, Andrew Cooper wrote:
> >> On 25/10/2023 9:52 pm, Pawan Gupta wrote:
> >>> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> >>> index bfb7bcb362bc..f8ba0c0b6e60 100644
> >>> --- a/arch/x86/entry/entry.S
> >>> +++ b/arch/x86/entry/entry.S
> >>> @@ -20,3 +23,16 @@ SYM_FUNC_END(entry_ibpb)
> >>>  EXPORT_SYMBOL_GPL(entry_ibpb);
> >>>  
> >>>  .popsection
> >>> +
> >>> +.pushsection .entry.text, "ax"
> >>> +
> >>> +.align L1_CACHE_BYTES, 0xcc
> >>> +SYM_CODE_START_NOALIGN(mds_verw_sel)
> >>> +	UNWIND_HINT_UNDEFINED
> >>> +	ANNOTATE_NOENDBR
> >>> +	.word __KERNEL_DS
> >> You need another .align here.  Otherwise subsequent code will still
> >> start in this cacheline and defeat the purpose of trying to keep it
> >> separate.
> >>
> >>> +SYM_CODE_END(mds_verw_sel);
> >> Thinking about it, should this really be CODE and not a data entry?
> >>
> >> It lives in .entry.text but it really is data and objtool shouldn't be
> >> writing ORC data for it at all.
> >>
> >> (Not to mention that if it's marked as STT_OBJECT, objdump -d will do
> >> the sensible thing and not even try to disassemble it).
> >>
> >> ~Andrew
> >>
> >> P.S. Please CC on the full series.  Far less effort than fishing the
> >> rest off lore.
> > +1 to putting it in .rodata or so.
> 
> It's necessarily in .entry.text so it doesn't explode with KPTI active.

Ah, right.  In general tooling doesn't take too kindly to putting data
in a text section.  But it might be ok.

-- 
Josh