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SubjectRe: Re: [PATCH] x86: set Pentium M as PAE capable
On Mon, Mar 03, 2014 at 03:04:35PM +0700, Chris Bainbridge wrote:
> On 3 March 2014 02:05, Roland Kletzing <devzero@web.de> wrote:
> > i would recommend adding the newly introduced param to
> > Documentation/kernel-
> > parameters.txt , though.
>
> Done.
>
> Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
> ---
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index b9e9bd8..388b5e9 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -962,6 +962,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> parameter will force ia64_sal_cache_flush to call
> ia64_pal_cache_flush instead of SAL_CACHE_FLUSH.
>
> + forcepae [X86-32]
> + Forcefully enable Physical Address Extension (PAE).
> + Many Pentium M systems disable PAE but may have a
> + functionally usable PAE implementation.
> + Note: This parameter is unsupported, may cause unknown

What does "unsupported" mean here exactly?

> + problems, and will taint the kernel.
> +
> ftrace=[tracer]
> [FTRACE] will set and start the specified tracer
> as early as possible in order to facilitate early
> diff --git a/arch/x86/boot/cpucheck.c b/arch/x86/boot/cpucheck.c
> index 4d3ff03..93ba160 100644
> --- a/arch/x86/boot/cpucheck.c
> +++ b/arch/x86/boot/cpucheck.c
> @@ -69,6 +69,13 @@ static int is_transmeta(void)
> cpu_vendor[2] == A32('M', 'x', '8', '6');
> }
>
> +static int is_intel(void)
> +{
> + return cpu_vendor[0] == A32('G', 'e', 'n', 'u') &&
> + cpu_vendor[1] == A32('i', 'n', 'e', 'I') &&
> + cpu_vendor[2] == A32('n', 't', 'e', 'l');
> +}
> +
> static int has_fpu(void)
> {
> u16 fcw = -1, fsw = -1;
> @@ -239,6 +246,24 @@ int check_cpu(int *cpu_level_ptr, int *req_level_ptr, u32 **err_flags_ptr)
> asm("wrmsr" : : "a" (eax), "d" (edx), "c" (ecx));
>
> err = check_flags();
> + } else if (err == 0x01 &&
> + !(err_flags[0] & ~(1 << X86_FEATURE_PAE)) &&
> + is_intel() && cpu.level == 6 &&
> + (cpu.model == 9 || cpu.model == 13)) {
> + /* PAE is disabled on this Pentium M but can be forced */
> + if (cmdline_find_option_bool("forcepae")) {
> + puts("WARNING: Forcing PAE in CPU flags\n");
> + set_bit(X86_FEATURE_PAE, cpu.flags);
> + err = check_flags();

This function is called check_cpuflags() now. You probably want to redo
your patch against tip/master, i.e.:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git#master

> + }
> + else {
> + puts("ERROR: PAE is disabled on this Pentium M\n"
> + "(PAE can potentially be enabled with "
> + "kernel parameter\n"
> + "\"forcepae\" - this is unsupported, may "
> + "cause unknown\n"
> + "problems, and will taint the kernel)\n");

This string could definitely violate the 80 cols rule so that it is much
more readable:

}
else
puts("WARNING: PAE disabled. Use \"forcepae\" to enable at your own risk!\n");

I've shortened it to the most relevant info only. No need to say we're
tainting the kernel because LOCKDEP_NOW_UNRELIABLE will cause that
anyway below.

> + }
> }
>
> if (err_flags_ptr)
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index bbe1b8b..271686d 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -196,6 +196,14 @@ static void intel_smp_check(struct cpuinfo_x86 *c)
> }
> }
>
> +static int forcepae;
> +static int __init forcepae_setup(char *__unused)
> +{
> + forcepae = 1;
> + return 1;
> +}
> +__setup("forcepae", forcepae_setup);

Yeah, why not simply call it "pae"? It is smaller and the letter
combination is not used yet and it means the same.

> +
> static void intel_workarounds(struct cpuinfo_x86 *c)
> {
> unsigned long lo, hi;
> @@ -226,6 +234,17 @@ static void intel_workarounds(struct cpuinfo_x86 *c)
> clear_cpu_cap(c, X86_FEATURE_SEP);
>
> /*
> + * PAE CPUID issue: many Pentium M report no PAE but may have a
> + * functionally usable PAE implementation.
> + * Forcefully enable PAE if kernel parameter "forcepae" is present.
> + */
> + if (forcepae) {
> + printk(KERN_WARNING "PAE forced!\n");
> + set_cpu_cap(c, X86_FEATURE_PAE);
> + add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_NOW_UNRELIABLE);

Right, this implies Dave's patch is preceding yours. I guess hpa can
fish it out from the thread when applying.

--
Regards/Gruss,
Boris.

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

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