[PATCH v3 02/10] KVM: selftests: Explicitly require instructions bytes

David Matlack
2022-10-31T18:00:57Z

Hard-code the flds instruction and assert the exact instruction bytes
are present in run->emulation_failure. The test already requires the
instruction bytes to be present because that's the only way the test
will advance the RIP past the flds and get to GUEST_DONE().

Note that KVM does not necessarily return exactly 2 bytes in
run->emulation_failure since it may not know the exact instruction
length in all cases. So just assert that
run->emulation_failure.insn_size is at least 2.

Signed-off-by: David Matlack <dmatlack@google.com>
---
.../smaller_maxphyaddr_emulation_test.c | 68 ++++++-------------
1 file changed, 20 insertions(+), 48 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/smaller_maxphyaddr_emulation_test.c b/tools/testing/selftests/kvm/x86_64/smaller_maxphyaddr_emulation_test.c
index 6ed996988a5a..d92cd4139f6d 100644
--- a/tools/testing/selftests/kvm/x86_64/smaller_maxphyaddr_emulation_test.c
+++ b/tools/testing/selftests/kvm/x86_64/smaller_maxphyaddr_emulation_test.c
@@ -19,41 +19,20 @@
#define MEM_REGION_SLOT 10
#define MEM_REGION_SIZE PAGE_SIZE

+#define FLDS_MEM_EAX ".byte 0xd9, 0x00"
+
static void guest_code(void)
{
- __asm__ __volatile__("flds (%[addr])"
- :: [addr]"r"(MEM_REGION_GVA));
+ __asm__ __volatile__(FLDS_MEM_EAX :: "a"(MEM_REGION_GVA));

GUEST_DONE();
}

-/*
- * Accessors to get R/M, REG, and Mod bits described in the SDM vol 2,
- * figure 2-2 "Table Interpretation of ModR/M Byte (C8H)".
- */
-#define GET_RM(insn_byte) (insn_byte & 0x7)
-#define GET_REG(insn_byte) ((insn_byte & 0x38) >> 3)
-#define GET_MOD(insn_byte) ((insn_byte & 0xc) >> 6)
-
-/* Ensure we are dealing with a simple 2-byte flds instruction. */
-static bool is_flds(uint8_t *insn_bytes, uint8_t insn_size)
-{
- return insn_size >= 2 &&
- insn_bytes[0] == 0xd9 &&
- GET_REG(insn_bytes[1]) == 0x0 &&
- GET_MOD(insn_bytes[1]) == 0x0 &&
- /* Ensure there is no SIB byte. */
- GET_RM(insn_bytes[1]) != 0x4 &&
- /* Ensure there is no displacement byte. */
- GET_RM(insn_bytes[1]) != 0x5;
-}
-
static void process_exit_on_emulation_error(struct kvm_vcpu *vcpu)
{
struct kvm_run *run = vcpu->run;
struct kvm_regs regs;
uint8_t *insn_bytes;
- uint8_t insn_size;
uint64_t flags;

TEST_ASSERT(run->exit_reason == KVM_EXIT_INTERNAL_ERROR,
@@ -65,30 +44,23 @@ static void process_exit_on_emulation_error(struct kvm_vcpu *vcpu)
"Unexpected suberror: %u",
run->emulation_failure.suberror);

- if (run->emulation_failure.ndata >= 1) {
- flags = run->emulation_failure.flags;
- if ((flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES) &&
- run->emulation_failure.ndata >= 3) {
- insn_size = run->emulation_failure.insn_size;
- insn_bytes = run->emulation_failure.insn_bytes;
-
- TEST_ASSERT(insn_size <= 15 && insn_size > 0,
- "Unexpected instruction size: %u",
- insn_size);
-
- TEST_ASSERT(is_flds(insn_bytes, insn_size),
- "Unexpected instruction. Expected 'flds' (0xd9 /0)");
-
- /*
- * If is_flds() succeeded then the instruction bytes
- * contained an flds instruction that is 2-bytes in
- * length (ie: no prefix, no SIB, no displacement).
- */
- vcpu_regs_get(vcpu, &regs);
- regs.rip += 2;
- vcpu_regs_set(vcpu, &regs);
- }
- }
+ flags = run->emulation_failure.flags;
+ TEST_ASSERT(run->emulation_failure.ndata >= 3 &&
+ flags & KVM_INTERNAL_ERROR_EMULATION_FLAG_INSTRUCTION_BYTES,
+ "run->emulation_failure is missing instruction bytes");
+
+ TEST_ASSERT(run->emulation_failure.insn_size >= 2,
+ "Expected a 2-byte opcode for 'flds', got %d bytes",
+ run->emulation_failure.insn_size);
+
+ insn_bytes = run->emulation_failure.insn_bytes;
+ TEST_ASSERT(insn_bytes[0] == 0xd9 && insn_bytes[1] == 0,
+ "Expected 'flds [eax]', opcode '0xd9 0x00', got opcode 0x%02x 0x%02x\n",
+ insn_bytes[0], insn_bytes[1]);
+
+ vcpu_regs_get(vcpu, &regs);
+ regs.rip += 2;
+ vcpu_regs_set(vcpu, &regs);
}

static void do_guest_assert(struct ucall *uc)
--
2.38.1.273.g43a17bfeac-goog