[PATCH 4/4] powerpc/64s: Support shrinking the SLB for debugging

[Michael Ellerman]

On machines with 1TB segments and a 32-entry SLB it's quite hard to
cause sufficient SLB pressure to trigger bugs caused due to badly
timed SLB faults.

We have seen this in the past and a few years ago added the
disable_1tb_segments command line option to force the use of 256MB
segments. However even this allows some bugs to slip through testing
if the SLB entry in question was recently accessed.

So add a new command line parameter for debugging which shrinks the
SLB to the minimum size we can support. Currently that size is 3, two
bolted SLBs and 1 for dynamic use. This creates the maximal SLB
pressure while still allowing the kernel to operate.

Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
---
 arch/powerpc/mm/slb.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index 61450a9cf30d..0f33e28f97da 100644
--- a/arch/powerpc/mm/slb.c
+++ b/arch/powerpc/mm/slb.c
@@ -506,10 +506,24 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
 	asm volatile("isync" : : : "memory");
 }
 
+static bool shrink_slb = false;
+
+static int __init parse_shrink_slb(char *p)
+{
+	shrink_slb = true;
+	slb_set_size(0);
+
+	return 0;
+}
+early_param("shrink_slb", parse_shrink_slb);
+
 static u32 slb_full_bitmap;
 
 void slb_set_size(u16 size)
 {
+	if (shrink_slb)
+		size = SLB_NUM_BOLTED + 1;
+
 	mmu_slb_size = size;
 
 	if (size >= 32)
-- 
2.20.1