Message ID | 20210113014047.14371-3-justin.he@arm.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | Avoid booting stall caused by | expand |
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
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();
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(+)