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    Subject[PATCH v7 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR
    Date
    This patch sets WT to the PA7 slot in the PAT MSR when the processor
    is not affected by the PAT errata. The PA7 slot is chosen to improve
    robustness in the presence of errata that might cause the high PAT bit
    to be ignored. This way a buggy PA7 slot access will hit the PA3 slot,
    which is UC, so at worst we lose performance without causing a correctness
    issue.

    The following Intel processors are affected by the PAT errata.

    errata cpuid
    ----------------------------------------------------
    Pentium 2, A52 family 0x6, model 0x5
    Pentium 3, E27 family 0x6, model 0x7, 0x8
    Pentium 3 Xenon, G26 family 0x6, model 0x7, 0x8, 0xa
    Pentium M, Y26 family 0x6, model 0x9
    Pentium M 90nm, X9 family 0x6, model 0xd
    Pentium 4, N46 family 0xf, model 0x0

    Instead of making sharp boundary checks, this patch makes conservative
    checks to exclude all Pentium 2, 3, M and 4 family processors. For
    such processors, _PAGE_CACHE_MODE_WT is redirected to UC- per the
    default setup in __cachemode2pte_tbl[].

    Signed-off-by: Toshi Kani <toshi.kani@hp.com>
    Reviewed-by: Juergen Gross <jgross@suse.com>
    ---
    arch/x86/mm/pat.c | 71 ++++++++++++++++++++++++++++++++++++++++++-----------
    1 file changed, 56 insertions(+), 15 deletions(-)

    diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
    index edf299c..3d9da74 100644
    --- a/arch/x86/mm/pat.c
    +++ b/arch/x86/mm/pat.c
    @@ -197,6 +197,7 @@ void pat_init(void)
    {
    u64 pat;
    bool boot_cpu = !boot_pat_state;
    + struct cpuinfo_x86 *c = &boot_cpu_data;

    if (!pat_enabled)
    return;
    @@ -217,21 +218,61 @@ void pat_init(void)
    }
    }

    - /* Set PWT to Write-Combining. All other bits stay the same */
    - /*
    - * PTE encoding used in Linux:
    - * PAT
    - * |PCD
    - * ||PWT
    - * |||
    - * 000 WB _PAGE_CACHE_WB
    - * 001 WC _PAGE_CACHE_WC
    - * 010 UC- _PAGE_CACHE_UC_MINUS
    - * 011 UC _PAGE_CACHE_UC
    - * PAT bit unused
    - */
    - pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
    - PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, UC);
    + if ((c->x86_vendor == X86_VENDOR_INTEL) &&
    + (((c->x86 == 0x6) && (c->x86_model <= 0xd)) ||
    + ((c->x86 == 0xf) && (c->x86_model <= 0x6)))) {
    + /*
    + * PAT support with the lower four entries. Intel Pentium 2,
    + * 3, M, and 4 are affected by PAT errata, which makes the
    + * upper four entries unusable. We do not use the upper four
    + * entries for all the affected processor families for safe.
    + *
    + * PTE encoding used in Linux:
    + * PAT
    + * |PCD
    + * ||PWT PAT
    + * ||| slot
    + * 000 0 WB : _PAGE_CACHE_MODE_WB
    + * 001 1 WC : _PAGE_CACHE_MODE_WC
    + * 010 2 UC-: _PAGE_CACHE_MODE_UC_MINUS
    + * 011 3 UC : _PAGE_CACHE_MODE_UC
    + * PAT bit unused
    + *
    + * NOTE: When WT or WP is used, it is redirected to UC- per
    + * the default setup in __cachemode2pte_tbl[].
    + */
    + pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
    + PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, UC);
    + } else {
    + /*
    + * PAT full support. We put WT in slot 7 to improve
    + * robustness in the presence of errata that might cause
    + * the high PAT bit to be ignored. This way a buggy slot 7
    + * access will hit slot 3, and slot 3 is UC, so at worst
    + * we lose performance without causing a correctness issue.
    + * Pentium 4 erratum N46 is an example of such an erratum,
    + * although we try not to use PAT at all on affected CPUs.
    + *
    + * PTE encoding used in Linux:
    + * PAT
    + * |PCD
    + * ||PWT PAT
    + * ||| slot
    + * 000 0 WB : _PAGE_CACHE_MODE_WB
    + * 001 1 WC : _PAGE_CACHE_MODE_WC
    + * 010 2 UC-: _PAGE_CACHE_MODE_UC_MINUS
    + * 011 3 UC : _PAGE_CACHE_MODE_UC
    + * 100 4 WB : Reserved
    + * 101 5 WC : Reserved
    + * 110 6 UC-: Reserved
    + * 111 7 WT : _PAGE_CACHE_MODE_WT
    + *
    + * The reserved slots are unused, but mapped to their
    + * corresponding types in the presence of PAT errata.
    + */
    + pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
    + PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, WT);
    + }

    /* Boot CPU check */
    if (!boot_pat_state)

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