powerpc: Dynamically allocate pacas

On 64-bit kernels we currently have a 512 byte struct paca_struct for
each cpu (usually just called "the paca"). Currently they are statically
allocated, which means a kernel built for a large number of cpus will
waste a lot of space if it's booted on a machine with few cpus.

We can avoid that by only allocating the number of pacas we need at
boot. However this is complicated by the fact that we need to access
the paca before we know how many cpus there are in the system.

The solution is to dynamically allocate enough space for NR_CPUS pacas,
but then later in boot when we know how many cpus we have, we free any
unused pacas.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index d8a693109c82..a011603d4079 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -14,6 +14,9 @@
 #define _ASM_POWERPC_PACA_H
 #ifdef __KERNEL__
 
+#ifdef CONFIG_PPC64
+
+#include <linux/init.h>
 #include <asm/types.h>
 #include <asm/lppaca.h>
 #include <asm/mmu.h>
@@ -145,8 +148,19 @@ struct paca_struct {
 #endif
 };
 
-extern struct paca_struct paca[];
-extern void initialise_pacas(void);
+extern struct paca_struct *paca;
+extern __initdata struct paca_struct boot_paca;
+extern void initialise_paca(struct paca_struct *new_paca, int cpu);
+
+extern void allocate_pacas(void);
+extern void free_unused_pacas(void);
+
+#else /* CONFIG_PPC64 */
+
+static inline void allocate_pacas(void) { };
+static inline void free_unused_pacas(void) { };
+
+#endif /* CONFIG_PPC64 */
 
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_PACA_H */