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Subject[PATCH 2/2] KVM: arm64: selftests: Disable single-step without relying on ucall()
From
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 VM-Exit for the debug
exception.

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>
---
.../selftests/kvm/aarch64/debug-exceptions.c | 28 ++++++++++---------
1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/kvm/aarch64/debug-exceptions.c b/tools/testing/selftests/kvm/aarch64/debug-exceptions.c
index 91f55b45dfc7..65cd7e4f07fa 100644
--- a/tools/testing/selftests/kvm/aarch64/debug-exceptions.c
+++ b/tools/testing/selftests/kvm/aarch64/debug-exceptions.c
@@ -241,7 +241,6 @@ static void guest_svc_handler(struct ex_regs *regs)

enum single_step_op {
SINGLE_STEP_ENABLE = 0,
- SINGLE_STEP_DISABLE = 1,
};

static void guest_code_ss(int test_cnt)
@@ -258,7 +257,7 @@ static void guest_code_ss(int test_cnt)
GUEST_SYNC(SINGLE_STEP_ENABLE);

/*
- * The userspace will veriry that the pc is as expected during
+ * The userspace will verify that the pc is as expected during
* single step execution between iter_ss_begin and iter_ss_end.
*/
asm volatile("iter_ss_begin:nop\n");
@@ -268,11 +267,9 @@ static void guest_code_ss(int test_cnt)
bvr = read_sysreg(dbgbvr0_el1);
wvr = read_sysreg(dbgwvr0_el1);

+ /* Userspace disables Single Step when the end is nigh. */
asm volatile("iter_ss_end:\n");

- /* Disable Single Step execution */
- GUEST_SYNC(SINGLE_STEP_DISABLE);
-
GUEST_ASSERT(bvr == w_bvr);
GUEST_ASSERT(wvr == w_wvr);
}
@@ -364,15 +361,12 @@ void test_single_step_from_userspace(int test_cnt)
TEST_ASSERT(cmd == UCALL_SYNC,
"Unexpected ucall cmd 0x%lx", cmd);

- if (uc.args[1] == SINGLE_STEP_ENABLE) {
- debug.control = KVM_GUESTDBG_ENABLE |
- KVM_GUESTDBG_SINGLESTEP;
- ss_enable = true;
- } else {
- debug.control = KVM_GUESTDBG_ENABLE;
- ss_enable = false;
- }
+ TEST_ASSERT(uc.args[1] == SINGLE_STEP_ENABLE,
+ "Unexpected ucall action 0x%lx", uc.args[1]);

+ debug.control = KVM_GUESTDBG_ENABLE |
+ KVM_GUESTDBG_SINGLESTEP;
+ ss_enable = true;
vcpu_guest_debug_set(vcpu, &debug);
continue;
}
@@ -385,6 +379,14 @@ void test_single_step_from_userspace(int test_cnt)
"Unexpected pc 0x%lx (expected 0x%lx)",
pc, test_pc);

+ if ((pc + 4) == (uint64_t)&iter_ss_end) {
+ test_pc = 0;
+ debug.control = KVM_GUESTDBG_ENABLE;
+ ss_enable = false;
+ vcpu_guest_debug_set(vcpu, &debug);
+ continue;
+ }
+
/*
* If the current pc is between iter_ss_bgin and
* iter_ss_end, the pc for the next KVM_EXIT_DEBUG should
--
2.38.1.431.g37b22c650d-goog
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