RE: [RFC PATCH 0/2] Avoid booting stall caused byidmap_kpti_install_ng_mappings

[Justin He]

Hi,
Kindly ping 😊

> -----Original Message-----
> From: Jia He <justin.he@xxxxxxx>
> Sent: Wednesday, January 13, 2021 9:41 AM
> To: Catalin Marinas <Catalin.Marinas@xxxxxxx>; Will Deacon
> <will@xxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Cc: Anshuman Khandual <Anshuman.Khandual@xxxxxxx>; Suzuki Poulose
> <Suzuki.Poulose@xxxxxxx>; Justin He <Justin.He@xxxxxxx>; Mark Rutland
> <Mark.Rutland@xxxxxxx>; Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>;
> Richard Henderson <richard.henderson@xxxxxxxxxx>; Dave P Martin
> <Dave.Martin@xxxxxxx>; Steven Price <Steven.Price@xxxxxxx>; Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxx>; Ard
> Biesheuvel <ardb@xxxxxxxxxx>; Gavin Shan <gshan@xxxxxxxxxx>; Kefeng Wang
> <wangkefeng.wang@xxxxxxxxxx>; Mark Brown <broonie@xxxxxxxxxx>; Marc Zyngier
> <maz@xxxxxxxxxx>; Cristian Marussi <Cristian.Marussi@xxxxxxx>
> Subject: [RFC PATCH 0/2] Avoid booting stall caused by
> 
> 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