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KVM: selftests: do not blindly clobber registers in guest asm
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The guest_code of sync_regs_test is assuming that the compiler will not
touch %r11 outside the asm that increments it, which is a bit brittle.
Instead, we can increment a variable and use a dummy asm to ensure the
increment is not optimized away.  However, we also need to use a
callee-save register or the compiler will insert a save/restore around
the vmexit, breaking the whole idea behind the test.

(Yes, "if it ain't broken...", but I would like the test to be clean
before it is copied into the upcoming s390 selftests).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Paolo Bonzini committed May 24, 2019
1 parent 12e9612 commit 204c91e
Showing 1 changed file with 30 additions and 24 deletions.
54 changes: 30 additions & 24 deletions tools/testing/selftests/kvm/x86_64/sync_regs_test.c
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Original file line number Diff line number Diff line change
Expand Up @@ -25,9 +25,15 @@

void guest_code(void)
{
/*
* use a callee-save register, otherwise the compiler
* saves it around the call to GUEST_SYNC.
*/
register u32 stage asm("rbx");
for (;;) {
GUEST_SYNC(0);
asm volatile ("inc %r11");
stage++;
asm volatile ("" : : "r" (stage));
}
}

Expand Down Expand Up @@ -147,7 +153,7 @@ int main(int argc, char *argv[])
compare_vcpu_events(&events, &run->s.regs.events);

/* Set and verify various register values. */
run->s.regs.regs.r11 = 0xBAD1DEA;
run->s.regs.regs.rbx = 0xBAD1DEA;
run->s.regs.sregs.apic_base = 1 << 11;
/* TODO run->s.regs.events.XYZ = ABC; */

Expand All @@ -158,9 +164,9 @@ int main(int argc, char *argv[])
"Unexpected exit reason: %u (%s),\n",
run->exit_reason,
exit_reason_str(run->exit_reason));
TEST_ASSERT(run->s.regs.regs.r11 == 0xBAD1DEA + 1,
"r11 sync regs value incorrect 0x%llx.",
run->s.regs.regs.r11);
TEST_ASSERT(run->s.regs.regs.rbx == 0xBAD1DEA + 1,
"rbx sync regs value incorrect 0x%llx.",
run->s.regs.regs.rbx);
TEST_ASSERT(run->s.regs.sregs.apic_base == 1 << 11,
"apic_base sync regs value incorrect 0x%llx.",
run->s.regs.sregs.apic_base);
Expand All @@ -179,57 +185,57 @@ int main(int argc, char *argv[])
*/
run->kvm_valid_regs = TEST_SYNC_FIELDS;
run->kvm_dirty_regs = 0;
run->s.regs.regs.r11 = 0xDEADBEEF;
run->s.regs.regs.rbx = 0xDEADBEEF;
rv = _vcpu_run(vm, VCPU_ID);
TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
"Unexpected exit reason: %u (%s),\n",
run->exit_reason,
exit_reason_str(run->exit_reason));
TEST_ASSERT(run->s.regs.regs.r11 != 0xDEADBEEF,
"r11 sync regs value incorrect 0x%llx.",
run->s.regs.regs.r11);
TEST_ASSERT(run->s.regs.regs.rbx != 0xDEADBEEF,
"rbx sync regs value incorrect 0x%llx.",
run->s.regs.regs.rbx);

/* Clear kvm_valid_regs bits and kvm_dirty_bits.
* Verify s.regs values are not overwritten with existing guest values
* and that guest values are not overwritten with kvm_sync_regs values.
*/
run->kvm_valid_regs = 0;
run->kvm_dirty_regs = 0;
run->s.regs.regs.r11 = 0xAAAA;
regs.r11 = 0xBAC0;
run->s.regs.regs.rbx = 0xAAAA;
regs.rbx = 0xBAC0;
vcpu_regs_set(vm, VCPU_ID, &regs);
rv = _vcpu_run(vm, VCPU_ID);
TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
"Unexpected exit reason: %u (%s),\n",
run->exit_reason,
exit_reason_str(run->exit_reason));
TEST_ASSERT(run->s.regs.regs.r11 == 0xAAAA,
"r11 sync regs value incorrect 0x%llx.",
run->s.regs.regs.r11);
TEST_ASSERT(run->s.regs.regs.rbx == 0xAAAA,
"rbx sync regs value incorrect 0x%llx.",
run->s.regs.regs.rbx);
vcpu_regs_get(vm, VCPU_ID, &regs);
TEST_ASSERT(regs.r11 == 0xBAC0 + 1,
"r11 guest value incorrect 0x%llx.",
regs.r11);
TEST_ASSERT(regs.rbx == 0xBAC0 + 1,
"rbx guest value incorrect 0x%llx.",
regs.rbx);

/* Clear kvm_valid_regs bits. Verify s.regs values are not overwritten
* with existing guest values but that guest values are overwritten
* with kvm_sync_regs values.
*/
run->kvm_valid_regs = 0;
run->kvm_dirty_regs = TEST_SYNC_FIELDS;
run->s.regs.regs.r11 = 0xBBBB;
run->s.regs.regs.rbx = 0xBBBB;
rv = _vcpu_run(vm, VCPU_ID);
TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
"Unexpected exit reason: %u (%s),\n",
run->exit_reason,
exit_reason_str(run->exit_reason));
TEST_ASSERT(run->s.regs.regs.r11 == 0xBBBB,
"r11 sync regs value incorrect 0x%llx.",
run->s.regs.regs.r11);
TEST_ASSERT(run->s.regs.regs.rbx == 0xBBBB,
"rbx sync regs value incorrect 0x%llx.",
run->s.regs.regs.rbx);
vcpu_regs_get(vm, VCPU_ID, &regs);
TEST_ASSERT(regs.r11 == 0xBBBB + 1,
"r11 guest value incorrect 0x%llx.",
regs.r11);
TEST_ASSERT(regs.rbx == 0xBBBB + 1,
"rbx guest value incorrect 0x%llx.",
regs.rbx);

kvm_vm_free(vm);

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