MIPS: Fix machine check exception in kmap_coherent()

On an SMP system with cache aliases, the following sequence of events may
happen:

1) copy_user_highpage() runs on CPU0, invoking kmap_coherent() to create a
   temporary mapping in the fixmap region
2) copy_page() starts on CPU0
3) CPU1 sends CPU0 an IPI asking CPU0 to run local_r4k_flush_cache_page()
4) CPU0 takes the interrupt, interrupting copy_page()
5) local_r4k_flush_cache_page() on CPU0 calls kmap_coherent() again
6) The second invocation of kmap_coherent() on CPU0 tries to use the
   same fixmap virtual address that was being used by copy_user_highpage()
7) CPU0 throws a machine check exception for the TLB address conflict

Fixed by creating an extra set of fixmap entries for use in interrupt
handlers.  This prevents fixmap VA conflicts between copy_user_highpage()
running in user context, and local_r4k_flush_cache_page() invoked from an
SMP IPI.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/fixmap.h

@@ -48,9 +48,9 @@ enum fixed_addresses {
 #define FIX_N_COLOURS 8
 	FIX_CMAP_BEGIN,
 #ifdef CONFIG_MIPS_MT_SMTC
-	FIX_CMAP_END = FIX_CMAP_BEGIN + (FIX_N_COLOURS * NR_CPUS),
+	FIX_CMAP_END = FIX_CMAP_BEGIN + (FIX_N_COLOURS * NR_CPUS * 2),
 #else
-	FIX_CMAP_END = FIX_CMAP_BEGIN + FIX_N_COLOURS,
+	FIX_CMAP_END = FIX_CMAP_BEGIN + (FIX_N_COLOURS * 2),
 #endif
 #ifdef CONFIG_HIGHMEM
 	/* reserved pte's for temporary kernel mappings */

arch/mips/mm/init.c

@@ -27,6 +27,7 @@
 #include <linux/swap.h>
 #include <linux/proc_fs.h>
 #include <linux/pfn.h>
+#include <linux/hardirq.h>
 
 #include <asm/asm-offsets.h>
 #include <asm/bootinfo.h>
@@ -132,7 +133,10 @@ void *kmap_coherent(struct page *page, unsigned long addr)
 	inc_preempt_count();
 	idx = (addr >> PAGE_SHIFT) & (FIX_N_COLOURS - 1);
 #ifdef CONFIG_MIPS_MT_SMTC
-	idx += FIX_N_COLOURS * smp_processor_id();
+	idx += FIX_N_COLOURS * smp_processor_id() +
+		(in_interrupt() ? (FIX_N_COLOURS * NR_CPUS) : 0);
+#else
+	idx += in_interrupt() ? FIX_N_COLOURS : 0;
 #endif
 	vaddr = __fix_to_virt(FIX_CMAP_END - idx);
 	pte = mk_pte(page, PAGE_KERNEL);