KVM: arm64: Don't acquire RCU read lock for exclusive table walks

KVM: arm64: Don't acquire RCU read lock for exclusive table walks

Marek reported a BUG resulting from the recent parallel faults changes,
as the hyp stage-1 map walker attempted to allocate table memory while
holding the RCU read lock:

  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

Since the hyp stage-1 table walkers are serialized by kvm_hyp_pgd_mutex,
RCU protection really doesn't add anything. Don't acquire the RCU read
lock for an exclusive walk.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118182222.3932898-3-oliver.upton@linux.dev