linux-kernel - Re: [PATCH] x86/mce: work around an erratum on fast string copy instructions.

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Date:   Thu, 17 Feb 2022 17:30:57 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jue Wang <juew@...gle.com>
Cc:     Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] x86/mce: work around an erratum on fast string copy
 instructions.

On Wed, Feb 16, 2022 at 01:53:13PM -0800, Jue Wang wrote:
> Subject: Re: [PATCH] x86/mce: work around an erratum on fast string copy

When sending a new version of the patch, make sure you add it to the
subject so that I know which one is the newest:

[PATCH -v<INCREASING NUMBER>] x86/mce: Work around an erratum with fast string copy instructions

> +static noinstr bool quirk_skylake_repmov(void)
> +{
> +	u64 mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
> +	u64 misc_enable = __rdmsr(MSR_IA32_MISC_ENABLE);
> +
> +	// Only applies the quirk to local machine checks, i.e., no broadcast
> +	// sync is needed.
> +	if ((mcgstatus & MCG_STATUS_LMCES) &&
> +	    unlikely(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
> +		u64 mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
> +
> +		// The blob of logic below is checking for a software
> +		// recoverable data fetch error.
> +		if ((mc1_status &
> +		     (MCI_STATUS_VAL | MCI_STATUS_OVER | MCI_STATUS_UC | MCI_STATUS_EN |
> +		      MCI_STATUS_ADDRV | MCI_STATUS_MISCV | MCI_STATUS_PCC |
> +		      MCI_STATUS_AR | MCI_STATUS_S)) ==
> +		     (MCI_STATUS_VAL |                   MCI_STATUS_UC | MCI_STATUS_EN |
> +		      MCI_STATUS_ADDRV | MCI_STATUS_MISCV |
> +		      MCI_STATUS_AR | MCI_STATUS_S)) {
> +			msr_clear_bit(MSR_IA32_MISC_ENABLE,
> +				      MSR_IA32_MISC_ENABLE_FAST_STRING_BIT);

With CONFIG_KASAN=y and CONFIG_DEBUG_ENTRY=y:

vmlinux.o: warning: objtool: quirk_skylake_repmov()+0x4d: call to msr_clear_bit() leaves .noinstr.text section

You're going to have to use the mce_{rd,wr}msrl() routines.

> +			mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
> +			mce_wrmsrl(MSR_IA32_MCx_STATUS(1), 0);
> +			pr_err_once("Errata detected, disable fast string copy instructions.\n");
> +			return true;
> +		}
> +	}
> +	return false;
> +}

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette