linux-kernel - Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP

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Message-ID: <3328e53d-b0f2-4516-a6a6-51ca33642683@intel.com>
Date: Wed, 11 Sep 2024 17:22:59 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Xin Li (Intel)" <xin@...or.com>, linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 peterz@...radead.org, andrew.cooper3@...rix.com
Subject: Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH
 #CP

On 9/11/24 16:19, Xin Li (Intel) wrote:
> +/*
> + * The WFE (WAIT_FOR_ENDBRANCH) bit in the augmented CS of FRED stack frame is
> + * set to 1 in missing-ENDBRANCH #CP exceptions.

I think there's a bit of relatively irrelevant info in there.  For
instance, I don't think it's super important to mention that FRED is
involved and where the WFE bit is in memory.

FRED's involvement is kinda a no-brainer from the whole X86_FEATURE_FRED
thing, and if you're reading exception handler code and don't know that
'regs' is on the stack, this probably isn't the place to explain that.

> + * If the WFE bit is left as 1, the CPU will generate another missing-ENDBRANCH
> + * #CP because the indirect branch tracker will be set in the WAIT_FOR_ENDBRANCH
> + * state upon completion of the following ERETS instruction and the CPU will
> + * restart from the IP that just caused a previous missing-ENDBRANCH #CP.
> + *
> + * Clear the WFE bit to avoid dead looping due to the above reason.
> + */
> +static void ibt_clear_fred_wfe(struct pt_regs *regs)
> +{
> +	if (cpu_feature_enabled(X86_FEATURE_FRED))
> +		regs->fred_cs.wfe = 0;
> +}

Can I suggest a slightly different comment?

/*
 * WFE==1 (WAIT_FOR_ENDBRANCH) means that the CPU expects the next ERETS
 * to jump to an ENDBR instruction. If the ENDBR is missing, the CPU
 * raises a #CP.
 *
 * Clear WFE to avoid that #CP.
 *
 * Use this function in a #CP handler to effectively give the next
 * ERETS a free pass to ignore IBT for a single instruction.
 */

I think original comment really needs a "How do I use this?" sentence or
two.

A comment at the call site also wouldn't hurt:

 	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)){
 		regs->ax = 0;
+		/* Disable IBT enforcement for one exception return: */
+		ibt_clear_fred_wfe(regs);
 		return;
 	}

I'm finding it kinda hard to concisely differentiate between the
"disable IBT at one ERETS" and "disable IBT forever", but I hope this
sounds good to folks.