x86, cpu: Add forcepae parameter for booting PAE kernels on PAE-disabled Pentium M

Many Pentium M systems disable PAE but may have a functionally usable PAE
implementation. This adds the "forcepae" parameter which bypasses the boot
check for PAE, and sets the CPU as being PAE capable. Using this parameter
will taint the kernel with TAINT_CPU_OUT_OF_SPEC.

Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Link: http://lkml.kernel.org/r/20140307114040.GA4997@localhost
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/arch/x86/boot/cpucheck.c b/arch/x86/boot/cpucheck.c
index 100a9a10076a..f0d0b20fe149 100644
--- a/arch/x86/boot/cpucheck.c
+++ b/arch/x86/boot/cpucheck.c
@@ -67,6 +67,13 @@ 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');
+}
+
 /* Returns a bitmask of which words we have error bits in */
 static int check_cpuflags(void)
 {
@@ -153,6 +160,19 @@ 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_cpuflags();
+        } 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_cpuflags();
+                }
+                else {
+                        puts("WARNING: PAE disabled. Use parameter 'forcepae' to enable at your own risk!\n");
+                }
         }
 
         if (err_flags_ptr)