KVM Arm Device passthrough and linux-rt

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Hi,

While trying device passthrough on Linux-rt with KVM Arm, I had
the following splat.

[  363.410141] 000: BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:974
[  363.410150] 000: in_atomic(): 0, irqs_disabled(): 128, pid: 2916, name: qemu-system-aar
[  363.410153] 000: 4 locks held by qemu-system-aar/2916:
[  363.410157] 000:  #0: ffff8007bd248100 (&vcpu->mutex){+.+.}, at: kvm_vcpu_ioctl+0x70/0xae0
[  363.410171] 000:  #1: ffff8007bd1e2b20 (&kvm->irq_srcu){....}, at: kvm_notify_acked_irq+0x7c/0x300
[  363.410179] 000:  #2: ffff8007bd1e2b20 (&kvm->irq_srcu){....}, at: irqfd_resampler_ack+0x0/0xd8
[  363.410187] 000:  #3: ffff8007c2b27d28 (&ctx->wqh#2){+.+.}, at: eventfd_signal+0x24/0x78
[  363.410196] 000: irq event stamp: 4033894
[  363.410197] 000: hardirqs last  enabled at (4033893): [<ffff000011119400>] _raw_spin_unlock_irqrestore+0x88/0x90
[  363.410203] 000: hardirqs last disabled at (4033894): [<ffff0000100bed18>] kvm_arch_vcpu_ioctl_run+0x2a8/0xc08
[  363.410207] 000: softirqs last  enabled at (0): [<ffff0000100edde0>] copy_process.isra.1.part.2+0x8d8/0x1958
[  363.410212] 000: softirqs last disabled at (0): [<0000000000000000>]  (null)
[  363.410216] 000: CPU: 0 PID: 2916 Comm: qemu-system-aar Tainted: G        W       5.0.14-rt9-00013-g4b2a13c8a804 #84
[  363.410219] 000: Hardware name: AMD Seattle (Rev.B0) Development Board (Overdrive) (DT)
[  363.410221] 000: Call trace:
[  363.410222] 000:  dump_backtrace+0x0/0x158
[  363.410225] 000:  show_stack+0x14/0x20
[  363.410227] 000:  dump_stack+0xa0/0xd4
[  363.410230] 000:  ___might_sleep+0x16c/0x1f8
[  363.410234] 000:  rt_spin_lock+0x5c/0x70
[  363.410237] 000:  eventfd_signal+0x24/0x78
[  363.410238] 000:  irqfd_resampler_ack+0x94/0xd8
[  363.410241] 000:  kvm_notify_acked_irq+0xf8/0x300
[  363.410243] 000:  vgic_v2_fold_lr_state+0x174/0x1e0
[  363.410246] 000:  kvm_vgic_sync_hwstate+0x5c/0x2b8
[  363.410249] 000:  kvm_arch_vcpu_ioctl_run+0x624/0xc08
[  363.410250] 000:  kvm_vcpu_ioctl+0x3a0/0xae0
[  363.410252] 000:  do_vfs_ioctl+0xbc/0x910
[  363.410255] 000:  ksys_ioctl+0x78/0xa8
[  363.410257] 000:  __arm64_sys_ioctl+0x1c/0x28
[  363.410260] 000:  el0_svc_common+0x90/0x118
[  363.410263] 000:  el0_svc_handler+0x2c/0x80
[  363.410265] 000:  el0_svc+0x8/0xc

This is happening because vgic_v2_fold_lr_state() is expected
to be called with interrupt disabled. However, some of the path
(e.g eventfd) will take a spinlock.

The spinlock is from the waitqueue, so using a raw_spin_lock cannot
even be considered.

Do you have any input on how this could be solved?

Cheers,

-- 
Julien Grall
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