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SubjectRe: [PATCH 3/4] x86: disallow running with 32-bit PTEs to work around erratum
Date
> 
> 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.
>
> 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
> on this hardware, but this is the safe thing to do.
>
> ---

<jawoff>

Isn't this work from Mr. Tlb?
>
> b/arch/x86/boot/boot.h | 1 +
> b/arch/x86/boot/cpu.c | 2 ++
> b/arch/x86/boot/cpucheck.c | 32 ++++++++++++++++++++++++++++++++
> b/arch/x86/boot/cpuflags.c | 1 +
> b/arch/x86/boot/cpuflags.h | 1 +
> 5 files changed, 37 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 2016-07-01 \
> 10:42:07.302790241 -0700
> +++ b/arch/x86/boot/boot.h 2016-07-01 10:42:07.311790650 -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 2016-07-01 \
> 10:42:07.303790286 -0700
> +++ b/arch/x86/boot/cpu.c 2016-07-01 10:42:07.312790695 -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 2016-07-01 \
> 10:42:07.305790377 -0700
> +++ b/arch/x86/boot/cpucheck.c 2016-07-01 10:42:07.312790695 -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 +188,32 @@ int check_cpu(int *cpu_level_ptr, int *r
>
> return (cpu.level < req_level || err) ? -1 : 0;
> }
> +
> +int check_knl_erratum(void)

s/knl/xeon_knl/ ?
> +{
> + /*
> + * 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 soace 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 processor due\n"
> + "to a processor erratum. Use a 64-bit kernel, or PAE.\n\n");

Give processor name to the scared readers please.

> +
> + 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 2016-07-01 \
> 10:42:07.307790468 -0700
> +++ b/arch/x86/boot/cpuflags.c 2016-07-01 10:42:07.312790695 -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 2016-07-01 \
> 10:42:07.308790514 -0700
> +++ b/arch/x86/boot/cpuflags.h 2016-07-01 10:42:07.313790740 -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];
> };
> _
>


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