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 moving init_cpu_features() ahead of early_fixmap_init(), we can use
cpu_have_const_cap earlier than before. Hence we can avoid this stall
by updating arm64_use_ng_mappings.
After this patch series, it reduces the kernel boot time from 14.7s to
4.1s:
Before:
[ 14.757569] Freeing initrd memory: 60752K
After:
[ 4.138819] Freeing initrd memory: 60752K
Set it as RFC because I want to resolve any other points which I have
misconerned.
Jia He (2):
arm64/cpuinfo: Move init_cpu_features() ahead of early_fixmap_init()
arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping
arch/arm64/include/asm/cpu.h | 1 +
arch/arm64/kernel/cpuinfo.c | 13 ++++++++++---
arch/arm64/kernel/setup.c | 18 +++++++++++++-----
arch/arm64/kernel/smp.c | 3 +--
4 files changed, 25 insertions(+), 10 deletions(-)
--
2.17.1
