linux-kernel - [RFC PATCH 0/2] Avoid booting stall caused by

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Date:   Wed, 13 Jan 2021 09:40:45 +0800
From:   Jia He <justin.he@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Jia He <justin.he@....com>,
        Mark Rutland <mark.rutland@....com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Richard Henderson <richard.henderson@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Steven Price <steven.price@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Gavin Shan <gshan@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Cristian Marussi <cristian.marussi@....com>
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