Re: [PATCH v5 08/14] KVM: arm64: Protect stage-2 traversal with RCU

[Oliver Upton <oliver.upton@linux.dev>]

Hi Marek,

On Mon, Nov 14, 2022 at 03:29:14PM +0100, Marek Szyprowski wrote:
> This patch landed in today's linux-next (20221114) as commit 
> c3119ae45dfb ("KVM: arm64: Protect stage-2 traversal with RCU"). 
> Unfortunately it introduces a following warning:

Thanks for the bug report :) I had failed to test nVHE in the past few
revisions of this series.

> --->8---
> 
> kvm [1]: IPA Size Limit: 40 bits
> BUG: sleeping function called from invalid context at 
> include/linux/sched/mm.h:274
> in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
> preempt_count: 0, expected: 0
> RCU nest depth: 1, expected: 0
> 2 locks held by swapper/0/1:
>   #0: ffff80000a8a44d0 (kvm_hyp_pgd_mutex){+.+.}-{3:3}, at: 
> __create_hyp_mappings+0x80/0xc4
>   #1: ffff80000a927720 (rcu_read_lock){....}-{1:2}, at: 
> kvm_pgtable_walk+0x0/0x1f4
> CPU: 2 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc3+ #5918
> Hardware name: Raspberry Pi 3 Model B (DT)
> Call trace:
>   dump_backtrace.part.0+0xe4/0xf0
>   show_stack+0x18/0x40
>   dump_stack_lvl+0x8c/0xb8
>   dump_stack+0x18/0x34
>   __might_resched+0x178/0x220
>   __might_sleep+0x48/0xa0
>   prepare_alloc_pages+0x178/0x1a0
>   __alloc_pages+0x9c/0x109c
>   alloc_page_interleave+0x1c/0xc4
>   alloc_pages+0xec/0x160
>   get_zeroed_page+0x1c/0x44
>   kvm_hyp_zalloc_page+0x14/0x20
>   hyp_map_walker+0xd4/0x134
>   kvm_pgtable_visitor_cb.isra.0+0x38/0x5c
>   __kvm_pgtable_walk+0x1a4/0x220
>   kvm_pgtable_walk+0x104/0x1f4
>   kvm_pgtable_hyp_map+0x80/0xc4
>   __create_hyp_mappings+0x9c/0xc4
>   kvm_mmu_init+0x144/0x1cc
>   kvm_arch_init+0xe4/0xef4
>   kvm_init+0x3c/0x3d0
>   arm_init+0x20/0x30
>   do_one_initcall+0x74/0x400
>   kernel_init_freeable+0x2e0/0x350
>   kernel_init+0x24/0x130
>   ret_from_fork+0x10/0x20
> kvm [1]: Hyp mode initialized successfully
> 
> --->8----
> 
> I looks that more changes in the KVM code are needed to use RCU for that 
> code.

Right, the specific issue is that while the stage-2 walkers preallocate
any table memory they may need, the hyp walkers do not and allocate
inline.

As hyp stage-1 is protected by a spinlock there is no actual need for
RCU in that case. I'll post something later on today that addresses the
issue.

--
Thanks,
Oliver
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