Messages in this thread | | | Date | Tue, 26 Jan 2021 14:14:30 +0000 | From | Will Deacon <> | Subject | Re: [RFC PATCH 2/2] arm64: kpti: Update arm64_use_ng_mappings before pagetable mapping |
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On Wed, Jan 13, 2021 at 09:40:47AM +0800, Jia He wrote: > 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@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);
Are you sure it's safe to run the cpu feature initialisation code this early? For example, we haven't even parsed the command-line yet, so I think a fair amount of stuff will break.
Of course, you could also just pass "mitigations=off" if you want your performance back.
Will
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