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, 9 deletions

diff --git a/arch/powerpc/platforms/iseries/exception.S b/arch/powerpc/platforms/iseries/exception.S
index 5369653dcf6a..fba5bf915073 100644
--- a/arch/powerpc/platforms/iseries/exception.S
+++ b/arch/powerpc/platforms/iseries/exception.S
@@ -43,17 +43,14 @@ system_reset_iSeries:
         LOAD_REG_ADDR(r23, alpaca)
         li      r0,ALPACA_SIZE
         sub     r23,r13,r23
-        divdu   r23,r23,r0              /* r23 has cpu number */
-        LOAD_REG_ADDR(r13, paca)
-        mulli   r0,r23,PACA_SIZE
-        add     r13,r13,r0
-        mtspr   SPRN_SPRG_PACA,r13      /* Save it away for the future */
-        mfmsr   r24
-        ori     r24,r24,MSR_RI
-        mtmsrd  r24                     /* RI on */
-        mr      r24,r23
+        divdu   r24,r23,r0              /* r24 has cpu number */
         cmpwi   0,r24,0                 /* Are we processor 0? */
         bne     1f
+        LOAD_REG_ADDR(r13, boot_paca)
+        mtspr   SPRN_SPRG_PACA,r13      /* Save it away for the future */
+        mfmsr   r23
+        ori     r23,r23,MSR_RI
+        mtmsrd  r23                     /* RI on */
         b       .__start_initialization_iSeries /* Start up the first processor */
 1:      mfspr   r4,SPRN_CTRLF
         li      r5,CTRL_RUNLATCH        /* Turn off the run light */
@@ -86,6 +83,16 @@ system_reset_iSeries:
 #endif
 
 2:
+        /* Load our paca now that it's been allocated */
+        LOAD_REG_ADDR(r13, paca)
+        ld      r13,0(r13)
+        mulli   r0,r24,PACA_SIZE
+        add     r13,r13,r0
+        mtspr   SPRN_SPRG_PACA,r13      /* Save it away for the future */
+        mfmsr   r23
+        ori     r23,r23,MSR_RI
+        mtmsrd  r23                     /* RI on */
+
         HMT_LOW
 #ifdef CONFIG_SMP
         lbz     r23,PACAPROCSTART(r13)  /* Test if this processor