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    Subject[PATCH v2] x86: Fix code patching for paravirt-alternatives on 486
    As reported in <http://bugs.debian.org/511703> and
    <http://bugs.debian.org/515982>, kernels with paravirt-alternatives
    enabled crash in text_poke_early() on at least some 486-class
    processors.

    The problem is that text_poke_early() itself uses inline functions
    affected by paravirt-alternatives and so will modify instructions that
    have already been prefetched. Pentium and later processors will
    invalidate the prefetched instructions in this case, but 486-class
    processors do not.

    Change sync_core() to limit prefetching on 486-class (and 386-class)
    processors, and move the call to sync_core() above the call to the
    modifiable local_irq_restore().

    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    Second try, incorporating the jmp into sync_core().

    Also not signed as I know git has trouble with MIME.

    Ben.

    arch/x86/include/asm/processor.h | 16 +++++++++++++---
    arch/x86/kernel/alternative.c | 2 +-
    2 files changed, 14 insertions(+), 4 deletions(-)

    diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
    index c776826..2db56c5 100644
    --- a/arch/x86/include/asm/processor.h
    +++ b/arch/x86/include/asm/processor.h
    @@ -703,13 +703,23 @@ static inline void cpu_relax(void)
    rep_nop();
    }

    -/* Stop speculative execution: */
    +/* Stop speculative execution and prefetching of modified code. */
    static inline void sync_core(void)
    {
    int tmp;

    - asm volatile("cpuid" : "=a" (tmp) : "0" (1)
    - : "ebx", "ecx", "edx", "memory");
    +#if defined(CONFIG_M386) || defined(CONFIG_M486)
    + if (boot_cpu_data.x86 < 5)
    + /* There is no speculative execution.
    + * jmp is a barrier to prefetching. */
    + asm volatile("jmp 1f\n1:\n" ::: "memory");
    + else
    +#endif
    + /* cpuid is a barrier to speculative execution.
    + * Prefetched instructions are automatically
    + * invalidated when modified. */
    + asm volatile("cpuid" : "=a" (tmp) : "0" (1)
    + : "ebx", "ecx", "edx", "memory");
    }

    static inline void __monitor(const void *eax, unsigned long ecx,
    diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
    index 4869351..de7353c 100644
    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -498,8 +498,8 @@ static void *__init_or_module text_poke_early(void *addr, const void *opcode,
    unsigned long flags;
    local_irq_save(flags);
    memcpy(addr, opcode, len);
    - local_irq_restore(flags);
    sync_core();
    + local_irq_restore(flags);
    /* Could also do a CLFLUSH here to speed up CPU recovery; but
    that causes hangs on some VIA CPUs. */
    return addr;
    --
    1.6.3.3


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