linux-kernel - Re: [PATCH v2] x86/mce: work around an erratum on fast string copy instructions.

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Date:   Fri, 18 Feb 2022 16:07:09 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jue Wang <juew@...gle.com>, Tony Luck <tony.luck@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v2] x86/mce: work around an erratum on fast string copy
 instructions.

On Thu, Feb 17, 2022 at 05:32:09PM -0800, Jue Wang wrote:
> +static noinstr bool quirk_skylake_repmov(void)
> +{
> +	u64 mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
> +	u64 misc_enable = __rdmsr(MSR_IA32_MISC_ENABLE);
> +
> +	/*
> +	 * Apply the quirk only 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));
> +
> +		/* Check 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)) {
> +			misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
> +			__wrmsr(MSR_IA32_MISC_ENABLE,
> +				(u32)misc_enable, (u32)(misc_enable >> 32));

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

I actually really meant that.

And I went and simplified this a bit more so that it is more readable,
diff ontop.

Also, Tony, I think the clearing of MCG_STATUS should happen last.

---
diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index c1a41da99975..1741be9b9464 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -831,34 +831,35 @@ quirk_sandybridge_ifu(int bank, struct mce *m, struct pt_regs *regs)
  */
 static noinstr bool quirk_skylake_repmov(void)
 {
-	u64 mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
-	u64 misc_enable = __rdmsr(MSR_IA32_MISC_ENABLE);
+	u64 mcgstatus   = mce_rdmsrl(MSR_IA32_MCG_STATUS);
+	u64 misc_enable = mce_rdmsrl(MSR_IA32_MISC_ENABLE);
+	u64 mc1_status;
 
 	/*
 	 * Apply the quirk only to local machine checks, i.e., no broadcast
 	 * sync is needed.
 	 */
-	if ((mcgstatus & MCG_STATUS_LMCES) &&
-	    (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
-		u64 mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
-
-		/* Check 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)) {
-			misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
-			__wrmsr(MSR_IA32_MISC_ENABLE,
-				(u32)misc_enable, (u32)(misc_enable >> 32));
-			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;
-		}
+	if (!(mcgstatus & MCG_STATUS_LMCES) ||
+	    !(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING))
+		return false;
+
+	mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
+
+	/* Check 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)) {
+		misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
+		mce_wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
+		mce_wrmsrl(MSR_IA32_MCx_STATUS(1), 0);
+		pr_err_once("Erratum detected, disable fast string copy instructions.\n");
+		return true;
 	}
+
 	return false;
 }
 
@@ -1432,7 +1433,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
 		return unexpected_machine_check(regs);
 
 	if (mce_flags.skx_repmov_quirk && quirk_skylake_repmov())
-		return;
+		goto clear;
 
 	/*
 	 * Establish sequential order between the CPUs entering the machine
@@ -1576,6 +1577,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
 out:
 	instrumentation_end();
 
+clear:
 	mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
 }
 EXPORT_SYMBOL_GPL(do_machine_check);

-- 
Regards/Gruss,
    Boris.

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