[RFC PATCH 2/2] arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping

[Jia He]

There is a 10s stall in idmap_kpti_install_ng_mappings when kernel boots
on a Ampere EMAG server.

Commit f992b4dfd58b ("arm64: kpti: Add ->enable callback to remap
swapper using nG mappings") updates the nG bit runtime if kpti is required.
But things get worse if rodata=full in map_mem(). NO_BLOCK_MAPPINGS |
NO_CONT_MAPPINGS is required when creating pagetable mapping. Hence all
ptes are fully mapped in this case. On a Ampere EMAG server with 256G
memory(pagesize=4k), it causes the 10s stall.

After previous commit moving init_cpu_features(), we can use
cpu_have_const_cap earlier than before. Hence we can avoid this stall
by updating arm64_use_ng_mappings.

Signed-off-by: Jia He <justin.he at arm.com>
---
 arch/arm64/kernel/setup.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index e078ab068f3b..51098ceb7159 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -306,6 +306,10 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	/* Init the cpu feature codes for boot cpu */
 	cpuinfo_store_boot_cpu();
 
+	/* ARM64_UNMAP_KERNEL_AT_EL0 cap can be updated in cpuinfo_store_boot_cpu() */
+	if (!arm64_use_ng_mappings)
+		arm64_use_ng_mappings = cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
+
 	early_fixmap_init();
 	early_ioremap_init();
 
-- 
2.17.1