linux-kernel - Re: [PATCH v5] x86: Enable fast strings on Intel if BIOS hasn't already

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Date:	Wed, 1 May 2013 13:33:52 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Andrew Lutomirski <luto@....edu>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v5] x86: Enable fast strings on Intel if BIOS hasn't
 already

On Tue, Apr 30, 2013 at 10:46:00PM -0700, Andy Lutomirski wrote:
> From: Andrew Lutomirski <luto@....edu>
> 
> Intel SDM volume 3A, 8.4.2 says:
> 
>   Software can disable fast-string operation by clearing the
>   fast-string-enable bit (bit 0) of IA32_MISC_ENABLE MSR.
>   However, Intel recomments that system software always enable
>   fast-string operation.
> 
> The Intel DQ67SW board (with latest BIOS) disables fast string
> operations if TXT is enabled.  A Lenovo X220 disables it regardless
> of TXT setting.  I doubt I'm the only person with a dumb BIOS like
> this.

Hmm, interesting. So I have a x230 and it is enabled here:

# rdmsr -x 0x000001a0
850089

It could be some fast strings erratum like AAJ6 or BD3 (they have
different names for what apparently is the same erratum in different
docs). Simply search for "intel fast strings erratum" and sample the
first couple of pdfs to get an idea.

If this erratum is actually the case here, it has no fix according to
the docs (same core in different packages :)) and it looks like OEM
vendors want to be on the safe side by disabling fast strings. So, in
this case, if you force-enable it, you could risk forcing the erratum if
the conditions apply (crossing page boundary with inconsistent memory
types).

You could check whether the CPU revisions you have are affected by the
erratum.

> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
> 
> v4 was a almost two years ago, but I just noticed that this is still a problem.
> This is tested on v3.9.
> 
> https://patchwork.kernel.org/patch/1073972/
> 
> This is identical to v4 of this patch except that it uses wrmsrl_safe instead
> of wrmsr_safe.
> 
>  arch/x86/kernel/cpu/intel.c | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 1905ce9..a4a3ef2 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -29,6 +29,7 @@
>  static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
>  {
>  	u64 misc_enable;
> +	bool allow_fast_string = true;
>  
>  	/* Unmask CPUID levels if masked: */
>  	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
> @@ -119,10 +120,11 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
>  	 * (model 2) with the same problem.
>  	 */
>  	if (c->x86 == 15) {
> -		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +		allow_fast_string = false;
>  
> +		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
>  		if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING) {
> -			printk(KERN_INFO "kmemcheck: Disabling fast string operations\n");
> +			printk_once(KERN_INFO "kmemcheck: Disabling fast string operations\n");
>  
>  			misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
>  			wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> @@ -131,13 +133,28 @@ static void __cpuinit early_init_intel(struct cpuinfo_x86 *c)
>  #endif
>  
>  	/*
> -	 * If fast string is not enabled in IA32_MISC_ENABLE for any reason,
> -	 * clear the fast string and enhanced fast string CPU capabilities.
> +	 * If BIOS didn't enable fast string operation, try to enable
> +	 * it ourselves.  If that fails, then clear the fast string
> +	 * and enhanced fast string CPU capabilities.
>  	 */
>  	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
>  		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +
> +		if (allow_fast_string &&
> +		    !(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
> +			misc_enable |= MSR_IA32_MISC_ENABLE_FAST_STRING;
> +			wrmsrl_safe(MSR_IA32_MISC_ENABLE, misc_enable);
> +
> +			/* Re-read to make sure it stuck. */
> +			rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +
> +			if (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)
> +				printk_once(KERN_INFO FW_WARN "IA32_MISC_ENABLE.FAST_STRING_ENABLE was not set\n");

Nit:

Why this printk here? You say already below that we've failed enabling
fast strings.

> +		}
> +
>  		if (!(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
> -			printk(KERN_INFO "Disabled fast string operations\n");
> +			if (allow_fast_string)
> +				printk_once(KERN_INFO "Failed to enable fast string operations\n");
>  			setup_clear_cpu_cap(X86_FEATURE_REP_GOOD);
>  			setup_clear_cpu_cap(X86_FEATURE_ERMS);
>  		}
> -- 

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--
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