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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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