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SubjectRe: KVM guest sometimes failed to boot because of kernel stack overflow if KPTI is enabled on a hisilicon ARM64 platform.
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Hi Will,

On 2018/6/21 0:28, Will Deacon wrote:
> On Thu, Jun 21, 2018 at 12:25:05AM +0800, Wei Xu wrote:
>> Hi James,
>>
>> On 2018/6/20 23:54, James Morse wrote:
>>> Hi Wei,
>>>
>>> On 20/06/18 16:52, Wei Xu wrote:
>>>> On 2018/6/20 22:42, Will Deacon wrote:
>>>>> Hmm, I wonder if this is at all related to RAS, since we've just enabled
>>>>> that and if we take a fault whilst rewriting swapper then we're going to
>>>>> get stuck. What happens if you set CONFIG_ARM64_RAS_EXTN=n in the guest?
>>>> I will try it now.
>>> It's not just the Kconfig symbol, could you also revert:
>>>
>>> f751daa4f9d3 ("arm64: Unconditionally enable IESB on exception entry/return for
>>> firmware-first")
>>>
>>>
>>> (reverts and build cleanly on 4.17)
>> Thanks to point out this!
>> I have disabled CONFIG_ARM64_RAS_EXTN and reverted that commit.
>> But I still got the stack overflow issue sometimes.
>> Do you have more hint?
> [...]
>
>> [ 0.076797] pstate: 604003c5 (nZCv DAIF +PAN -UAO)
>> [ 0.081727] pc : el1_sync+0x0/0xb0
>> [ 0.085217] lr : kpti_install_ng_mappings+0x120/0x214
> Please run:
>
> $ ./scripts/faddr2line vmlinux kpti_install_ng_mappings+0x120/0x214

Thanks for your kindly guide :)
The output is as below:

joyx@Turing-Arch-b:~/plinth-kernel-v200$ ./scripts/faddr2line
../kernel-dev.build/vmlinux kpti_install_ng_mappings+0x120/0x214
kpti_install_ng_mappings+0x120/0x214:
cpu_set_reserved_ttbr0 at arch/arm64/include/asm/mmu_context.h:52
47 /*
48 * Set TTBR0 to empty_zero_page. No translations will be
possible via TTBR0.
49 */
50 static inline void cpu_set_reserved_ttbr0(void)
51 {
52 unsigned long ttbr =
phys_to_ttbr(__pa_symbol(empty_zero_page));
53
54 write_sysreg(ttbr, ttbr0_el1);
55 isb();
56 }
57
(inlined by) cpu_uninstall_idmap at
arch/arm64/include/asm/mmu_context.h:123
118 */
119 static inline void cpu_uninstall_idmap(void)
120 {
121 struct mm_struct *mm = current->active_mm;
122
123 cpu_set_reserved_ttbr0();
124 local_flush_tlb_all();
125 cpu_set_default_tcr_t0sz();
126
127 if (mm != &init_mm && !system_uses_ttbr0_pan())
128 cpu_switch_mm(mm->pgd, mm);
(inlined by) kpti_install_ng_mappings at
arch/arm64/kernel/cpufeature.c:922
917
918 remap_fn = (void
*)__pa_symbol(idmap_kpti_install_ng_mappings);
919
920 cpu_install_idmap();
921 remap_fn(cpu, num_online_cpus(),
__pa_symbol(swapper_pg_dir));
922 cpu_uninstall_idmap();
923
924 if (!cpu)
925 kpti_applied = true;
926
927 return;

Thanks!

Best Regards,
Wei

> as the GDB output wasn't helpful (it only showed local variable
> declarations?!).
>
> Will
>
> .
>


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