Re: [STABLE 5.15.y] [PATCH] KVM: arm64: Fix softirq masking in FPSIMD register saving sequence

[Will Deacon <will@kernel.org>]

On Fri, Oct 03, 2025 at 07:39:17PM +0100, Will Deacon wrote:
> Stable commit 23249dade24e ("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

Sorry, Greg, I lost a stray '>' here ^^^  and so you didn't end up on CC
for this one.

Will