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SubjectRe: selftests: ftrace: Internal error: Oops: sve_save_state
On Tue, Oct 17, 2023 at 01:34:18PM +0530, Naresh Kamboju wrote:

> Following kernel crash noticed while running selftests: ftrace:
> ftracetest-ktap on FVP models running stable-rc 6.5.8-rc2.

> This is not an easy to reproduce issue and not seen on mainline and next.
> We are investigating this report.

To confirm have you seen this on other stables as well or is this only
v6.5? For how long have you been seeing this?

> [ 764.987161] Unable to handle kernel NULL pointer dereference at
> virtual address 0000000000000000

> [ 765.074221] Call trace:
> [ 765.075045] sve_save_state+0x4/0xf0
> [ 765.076138] fpsimd_thread_switch+0x2c/0xe8
> [ 765.077305] __switch_to+0x20/0x158
> [ 765.078384] __schedule+0x2cc/0xb38
> [ 765.079464] preempt_schedule_irq+0x44/0xa8
> [ 765.080633] el1_interrupt+0x4c/0x68
> [ 765.081691] el1h_64_irq_handler+0x18/0x28
> [ 765.082829] el1h_64_irq+0x64/0x68
> [ 765.083874] ftrace_return_to_handler+0x98/0x158
> [ 765.085090] return_to_handler+0x20/0x48
> [ 765.086205] do_sve_acc+0x64/0x128
> [ 765.087272] el0_sve_acc+0x3c/0xa0
> [ 765.088356] el0t_64_sync_handler+0x114/0x130
> [ 765.089524] el0t_64_sync+0x190/0x198

So something managed to get flagged as having SVE state without having
the backing storage allocated. We *were* preempted in the SVE access
handler which does the allocation but I can't see the path that would
trigger that since we allocate the state before setting TIF_SVE. It's
possible the compiler did something funky, a decode of the backtrace
might help show that?
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