[PATCH 3/4] x86: disallow running with 32-bit PTEs to work around erratum

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

The Intel(R) Xeon Phi(TM) Processor x200 Family (codename: Knights
Landing) has an erratum where a processor thread setting the Accessed
or Dirty bits may not do so atomically against its checks for the
Present bit.  This may cause a thread (which is about to page fault)
to set A and/or D, even though the Present bit had already been
atomically cleared.

These bits are truly "stray".  In the case of the Dirty bit, the
thread associated with the stray set was *not* allowed to write to
the page.  This means that we do not have to launder the bit(s); we
can simply ignore them.

If the PTE is used for storing a swap index or a NUMA migration index,
the A bit could be misinterpreted as part of the swap type.  The stray
bits being set cause a software-cleared PTE to be interpreted as a
swap entry.  In some cases (like when the swap index ends up being
for a non-existent swapfile), the kernel detects the stray value
and WARN()s about it, but there is no guarantee that the kernel can
always detect it.

When we have 64-bit PTEs (64-bit mode or 32-bit PAE), we were able
to move the swap PTE format around to avoid these troublesome bits.
But, 32-bit non-PAE is tight on bits.  So, disallow it from running
on this hardware.  I can't imagine anyone wanting to run 32-bit
non-highmem kernels on this hardware, but disallowing them from
running entirely is surely the safe thing to do.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/arch/x86/boot/boot.h     |    1 +
 b/arch/x86/boot/cpu.c      |    2 ++
 b/arch/x86/boot/cpucheck.c |   33 +++++++++++++++++++++++++++++++++
 b/arch/x86/boot/cpuflags.c |    1 +
 b/arch/x86/boot/cpuflags.h |    1 +
 5 files changed, 38 insertions(+)

diff -puN arch/x86/boot/boot.h~knl-strays-40-disallow-non-PAE-32-bit-on-KNL arch/x86/boot/boot.h
--- a/arch/x86/boot/boot.h~knl-strays-40-disallow-non-PAE-32-bit-on-KNL	2016-07-07 17:17:44.420785026 -0700
+++ b/arch/x86/boot/boot.h	2016-07-07 17:17:44.430785476 -0700
@@ -295,6 +295,7 @@ static inline int cmdline_find_option_bo
 
 /* cpu.c, cpucheck.c */
 int check_cpu(int *cpu_level_ptr, int *req_level_ptr, u32 **err_flags_ptr);
+int check_knl_erratum(void);
 int validate_cpu(void);
 
 /* early_serial_console.c */
diff -puN arch/x86/boot/cpu.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL arch/x86/boot/cpu.c
--- a/arch/x86/boot/cpu.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL	2016-07-07 17:17:44.422785116 -0700
+++ b/arch/x86/boot/cpu.c	2016-07-07 17:17:44.430785476 -0700
@@ -93,6 +93,8 @@ int validate_cpu(void)
 		show_cap_strs(err_flags);
 		putchar('\n');
 		return -1;
+	} else if (check_knl_erratum()) {
+		return -1;
 	} else {
 		return 0;
 	}
diff -puN arch/x86/boot/cpucheck.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL arch/x86/boot/cpucheck.c
--- a/arch/x86/boot/cpucheck.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL	2016-07-07 17:17:44.423785161 -0700
+++ b/arch/x86/boot/cpucheck.c	2016-07-07 17:17:44.431785520 -0700
@@ -24,6 +24,7 @@
 # include "boot.h"
 #endif
 #include <linux/types.h>
+#include <asm/intel-family.h>
 #include <asm/processor-flags.h>
 #include <asm/required-features.h>
 #include <asm/msr-index.h>
@@ -175,6 +176,8 @@ int check_cpu(int *cpu_level_ptr, int *r
 			puts("WARNING: PAE disabled. Use parameter 'forcepae' to enable at your own risk!\n");
 		}
 	}
+	if (!err)
+		err = check_knl_erratum();
 
 	if (err_flags_ptr)
 		*err_flags_ptr = err ? err_flags : NULL;
@@ -185,3 +188,33 @@ int check_cpu(int *cpu_level_ptr, int *r
 
 	return (cpu.level < req_level || err) ? -1 : 0;
 }
+
+int check_knl_erratum(void)
+{
+	/*
+	 * First check for the affected model/family:
+	 */
+	if (!is_intel() ||
+	    cpu.family != 6 ||
+	    cpu.model != INTEL_FAM6_XEON_PHI_KNL)
+		return 0;
+
+	/*
+	 * This erratum affects the Accessed/Dirty bits, and can
+	 * cause stray bits to be set in !Present PTEs.  We have
+	 * enough bits in our 64-bit PTEs (which we have on real
+	 * 64-bit mode or PAE) to avoid using these troublesome
+	 * bits.  But, we do not have enough space in our 32-bit
+	 * PTEs.  So, refuse to run on 32-bit non-PAE kernels.
+	 */
+	if (IS_ENABLED(CONFIG_X86_64) || IS_ENABLED(CONFIG_X86_PAE))
+		return 0;
+
+	puts("This 32-bit kernel can not run on this Xeon Phi x200\n"
+	     "processor due to a processor erratum.  Use a 64-bit\n"
+	     "kernel, or enable PAE in this 32-bit kernel.\n\n");
+
+	return -1;
+}
+
+
diff -puN arch/x86/boot/cpuflags.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL arch/x86/boot/cpuflags.c
--- a/arch/x86/boot/cpuflags.c~knl-strays-40-disallow-non-PAE-32-bit-on-KNL	2016-07-07 17:17:44.425785251 -0700
+++ b/arch/x86/boot/cpuflags.c	2016-07-07 17:17:44.431785520 -0700
@@ -102,6 +102,7 @@ void get_cpuflags(void)
 			cpuid(0x1, &tfms, &ignored, &cpu.flags[4],
 			      &cpu.flags[0]);
 			cpu.level = (tfms >> 8) & 15;
+			cpu.family = cpu.level;
 			cpu.model = (tfms >> 4) & 15;
 			if (cpu.level >= 6)
 				cpu.model += ((tfms >> 16) & 0xf) << 4;
diff -puN arch/x86/boot/cpuflags.h~knl-strays-40-disallow-non-PAE-32-bit-on-KNL arch/x86/boot/cpuflags.h
--- a/arch/x86/boot/cpuflags.h~knl-strays-40-disallow-non-PAE-32-bit-on-KNL	2016-07-07 17:17:44.427785341 -0700
+++ b/arch/x86/boot/cpuflags.h	2016-07-07 17:17:44.431785520 -0700
@@ -6,6 +6,7 @@
 
 struct cpu_features {
 	int level;		/* Family, or 64 for x86-64 */
+	int family;		/* Family, always */
 	int model;
 	u32 flags[NCAPINTS];
 };
_