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SubjectRe: Re: [PATCH] x86: set Pentium M as PAE capable
On Sun, Mar 02, 2014 at 09:56:19PM +0100, Andreas Mohr wrote:
> Hi,
>
> > /*
> > + * PAE CPUID bug: Pentium M reports no PAE but has PAE
> > + */
>
> Ain't that a tad strongly/incorrectly worded?

I've updated the wording.

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
+ 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();
+ }
+ 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");
+ }
}

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);
+
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);
+ }
+
+ /*
* P4 Xeon errata 037 workaround.
* Hardware prefetcher may cause stale data to be loaded into the cache.
*/

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