linux-kernel - Re: [PATCH v3 1/6] x86/bugs: Add asm helpers for executing VERW

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Date:   Fri, 27 Oct 2023 06:48:29 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Andrew Cooper <andrew.cooper3@...rix.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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 v3 1/6] x86/bugs: Add asm helpers for executing VERW

On Wed, Oct 25, 2023 at 11:13:46PM +0100, Andrew Cooper wrote:
> On 25/10/2023 11:07 pm, Pawan Gupta wrote:
> > On Wed, Oct 25, 2023 at 10:10:41PM +0100, Andrew Cooper wrote:
> >>> +.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.
> > Right.
> >
> >>> +SYM_CODE_END(mds_verw_sel);
> >> Thinking about it, should this really be CODE and not a data entry?
> > Would that require adding a data equivalent of .entry.text and update
> > KPTI to keep it mapped? Or is there an easier option?
> 
> Leave it right here in .entry.text , but try using SYM_DATA() and
> friends.  See whether objtool vomits over the result or not.

objtool still complaints when using SYM_DATA*() without the annotations:

 vmlinux.o: warning: objtool: mds_verw_sel+0x0: unreachable instruction
 vmlinux.o: warning: objtool: .altinstr_replacement+0x2c: relocation to !ENDBR: mds_verw_sel+0x0

> And if objtool does vomit over the result, then leaving it as it is in
> this patch with SYM_CODE() is good enough.

Settling with SYM_CODE().

On the bright-side, I am seeing even better perf with VERW operand
out-of-line:

Baseline: v6.6-rc5

| Test               | Configuration          | v1   | v3   |
| ------------------ | ---------------------- | ---- | ---- |
| build-linux-kernel | defconfig              | 1.00 | 1.00 |
| hackbench          | 32 - Process           | 1.02 | 1.06 |
| nginx              | Short Connection - 500 | 1.01 | 1.04 |

Disclaimer: These are collected by a stupid dev who knows nothing about
perf, please take this with a grain of salt.

I will be sending v4 soon.