On Fri, 3 Oct 2025 at 20:41, Will Deacon [off-list ref] wrote:

Stable commit 28b82be094e2 ("KVM: arm64: Fix kernel BUG() due to bad
backport of FPSIMD/SVE/SME fix") fixed a kernel BUG() caused by a bad
backport of upstream commit fbc7e61195e2 ("KVM: arm64: Unconditionally
save+flush host FPSIMD/SVE/SME state") by ensuring that softirqs are
disabled/enabled across the fpsimd register save operation.

Unfortunately, although this fixes the original issue, it can now lead
to deadlock when re-enabling softirqs causes pending softirqs to be
handled with locks already held:

 | BUG: spinlock recursion on CPU#7, CPU 3/KVM/57616
 |  lock: 0xffff3045ef850240, .magic: dead4ead, .owner: CPU 3/KVM/57616, .owner_cpu: 7
 | CPU: 7 PID: 57616 Comm: CPU 3/KVM Tainted: G           O       6.1.152 #1
 | Hardware name: SoftIron SoftIron Platform Mainboard/SoftIron Platform Mainboard, BIOS 1.31 May 11 2023
 | Call trace:
 |  dump_backtrace+0xe4/0x110
 |  show_stack+0x20/0x30
 |  dump_stack_lvl+0x6c/0x88
 |  dump_stack+0x18/0x34
 |  spin_dump+0x98/0xac
 |  do_raw_spin_lock+0x70/0x128
 |  _raw_spin_lock+0x18/0x28
 |  raw_spin_rq_lock_nested+0x18/0x28
 |  update_blocked_averages+0x70/0x550
 |  run_rebalance_domains+0x50/0x70
 |  handle_softirqs+0x198/0x328
 |  __do_softirq+0x1c/0x28
 |  ____do_softirq+0x18/0x28
 |  call_on_irq_stack+0x30/0x48
 |  do_softirq_own_stack+0x24/0x30
 |  do_softirq+0x74/0x90
 |  __local_bh_enable_ip+0x64/0x80
 |  fpsimd_save_and_flush_cpu_state+0x5c/0x68
 |  kvm_arch_vcpu_put_fp+0x4c/0x88
 |  kvm_arch_vcpu_put+0x28/0x88
 |  kvm_sched_out+0x38/0x58
 |  __schedule+0x55c/0x6c8
 |  schedule+0x60/0xa8

Take a tiny step towards the upstream fix in 9b19700e623f ("arm64:
fpsimd: Drop unneeded 'busy' flag") by additionally disabling hardirqs
while saving the fpsimd registers.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Lee Jones <lee@kernel.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <redacted> # 6.6.y
Fixes: 28b82be094e2 ("KVM: arm64: Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix")
Reported-by: Kenneth Van Alstyne <redacted>
Link: https://lore.kernel.org/r/010001999bae0958-4d80d25d-8dda-4006-a6b9-798f3e774f6c-000000@email.amazonses.com (local)
Signed-off-by: Will Deacon <will@kernel.org>

Acked-by: Ard Biesheuvel <ardb@kernel.org>