KVM: arm64: selftests: Disable single-step without relying on ucall()
Automatically disable single-step when the guest reaches the end of the
verified section instead of using an explicit ucall() to ask userspace to
disable single-step. An upcoming change to implement a pool-based scheme
for ucall() will add an atomic operation (bit test and set) in the guest
ucall code, and if the compiler generate "old school" atomics, e.g.
40e57c:
c85f7c20 ldxr x0, [x1]
40e580:
aa100011 orr x17, x0, x16
40e584:
c80ffc31 stlxr w15, x17, [x1]
40e588:
35ffffaf cbnz w15, 40e57c <__aarch64_ldset8_sync+0x1c>
the guest will hang as the local exclusive monitor is reset by eret,
i.e. the stlxr will always fail due to the debug exception taken to EL2.
Link: https://lore.kernel.org/all/20221006003409.649993-8-seanjc@google.com
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221117002350.2178351-3-seanjc@google.com
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>