linux-kernel - kvm: arm64: stable commit "Fix kernel BUG() due to bad backport of FPSIMD/SVE/SME fix" deadlocks host kernel

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Message-ID: <hjc7jwarhmwrvcswa7nax26vgvs2wl7256pf3ba3onoaj26s5x@qubtqvi3vy6u>
Date: Wed, 8 Oct 2025 13:45:32 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, 
	Mark Brown <broonie@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Tomasz Figa <tfiga@...omium.org>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: kvm: arm64: stable commit "Fix kernel BUG() due to bad backport of
 FPSIMD/SVE/SME fix" deadlocks host kernel

Commits 8f4dc4e54eed4 (6.1.y) and 23249dade24e6 (5.15.y) (maybe other
stable kernels as well) deadlock the host kernel (presumably a
recursive spinlock):

 queued_spin_lock_slowpath+0x274/0x358
 raw_spin_rq_lock_nested+0x2c/0x48
 _raw_spin_rq_lock_irqsave+0x30/0x4c
 run_rebalance_domains+0x808/0x2e18
 __do_softirq+0x104/0x550
 irq_exit+0x88/0xe0
 handle_domain_irq+0x7c/0xb0
 gic_handle_irq+0x1cc/0x420
 call_on_irq_stack+0x20/0x48
 do_interrupt_handler+0x3c/0x50
 el1_interrupt+0x30/0x58
 el1h_64_irq_handler+0x18/0x24
 el1h_64_irq+0x7c/0x80
 kvm_arch_vcpu_ioctl_run+0x24c/0x49c
 kvm_vcpu_ioctl+0xc4/0x614

We found out a similar report at [1], but it doesn't seem like a formal
patch was ever posted.  Will, can you please send a formal patch so that
stable kernels can run VMs again?

[1] https://lists.linaro.org/archives/list/linux-stable-mirror@lists.linaro.org/thread/3FQHC4GVN57SM2CNST3EMVEBUXMSFOGR/#AQB4LMHLGTUO73GVCVV5QLCEJT3MRTN4