linux-kernel - [RFC PATCH 2/2] arm64: kpti: Update arm64_use_ng

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Date:   Wed, 13 Jan 2021 09:40:47 +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 2/2] arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping

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@....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