[PATCH v3 08/10] KVM: selftests: Provide error code as a KVM_ASM_SAFE() output

David Matlack
2022-10-31T18:01:14Z

From: Sean Christopherson <seanjc@google.com>

Provide the error code on a fault in KVM_ASM_SAFE(), e.g. to allow tests
to assert that #PF generates the correct error code without needing to
manually install a #PF handler. Use r10 as the scratch register for the
error code, as it's already clobbered by the asm blob (loaded with the
RIP of the to-be-executed instruction). Deliberately load the output
"error_code" even in the non-faulting path so that error_code is always
initialized with deterministic data (the aforementioned RIP), i.e to
ensure a selftest won't end up with uninitialized consumption regardless
of how KVM_ASM_SAFE() is used.

Don't clear r10 in the non-faulting case and instead load error code with
the RIP (see above). The error code is valid if and only if an exception
occurs, and '0' isn't necessarily a better "invalid" value, e.g. '0'
could result in false passes for a buggy test.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
.../selftests/kvm/include/x86_64/processor.h | 39 +++++++++++++------
.../selftests/kvm/lib/x86_64/processor.c | 1 +
.../selftests/kvm/x86_64/hyperv_features.c | 3 +-
3 files changed, 30 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
index 9efe80d52389..33b0f19e502c 100644
--- a/tools/testing/selftests/kvm/include/x86_64/processor.h
+++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
@@ -780,6 +780,7 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
*
* REGISTER OUTPUTS:
* r9 = exception vector (non-zero)
+ * r10 = error code
*/
#define KVM_ASM_SAFE(insn) \
"mov $" __stringify(KVM_EXCEPTION_MAGIC) ", %%r9\n\t" \
@@ -788,29 +789,43 @@ void vm_install_exception_handler(struct kvm_vm *vm, int vector,
"1: " insn "\n\t" \
"xor %%r9, %%r9\n\t" \
"2:\n\t" \
- "mov %%r9b, %[vector]\n\t"
+ "mov %%r9b, %[vector]\n\t" \
+ "mov %%r10, %[error_code]\n\t"

-#define KVM_ASM_SAFE_OUTPUTS(v) [vector] "=qm"(v)
+#define KVM_ASM_SAFE_OUTPUTS(v, ec) [vector] "=qm"(v), [error_code] "=rm"(ec)
#define KVM_ASM_SAFE_CLOBBERS "r9", "r10", "r11"

-#define kvm_asm_safe(insn, inputs...) \
-({ \
- uint8_t vector; \
- \
- asm volatile(KVM_ASM_SAFE(insn) \
- : KVM_ASM_SAFE_OUTPUTS(vector) \
- : inputs \
- : KVM_ASM_SAFE_CLOBBERS); \
- vector; \
+#define kvm_asm_safe(insn, inputs...) \
+({ \
+ uint64_t ign_error_code; \
+ uint8_t vector; \
+ \
+ asm volatile(KVM_ASM_SAFE(insn) \
+ : KVM_ASM_SAFE_OUTPUTS(vector, ign_error_code) \
+ : inputs \
+ : KVM_ASM_SAFE_CLOBBERS); \
+ vector; \
+})
+
+#define kvm_asm_safe_ec(insn, error_code, inputs...) \
+({ \
+ uint8_t vector; \
+ \
+ asm volatile(KVM_ASM_SAFE(insn) \
+ : KVM_ASM_SAFE_OUTPUTS(vector, error_code) \
+ : inputs \
+ : KVM_ASM_SAFE_CLOBBERS); \
+ vector; \
})

static inline uint8_t rdmsr_safe(uint32_t msr, uint64_t *val)
{
+ uint64_t error_code;
uint8_t vector;
uint32_t a, d;

asm volatile(KVM_ASM_SAFE("rdmsr")
- : "=a"(a), "=d"(d), KVM_ASM_SAFE_OUTPUTS(vector)
+ : "=a"(a), "=d"(d), KVM_ASM_SAFE_OUTPUTS(vector, error_code)
: "c"(msr)
: KVM_ASM_SAFE_CLOBBERS);

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index 39c4409ef56a..fc6c724e0d24 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -1116,6 +1116,7 @@ static bool kvm_fixup_exception(struct ex_regs *regs)

regs->rip = regs->r11;
regs->r9 = regs->vector;
+ regs->r10 = regs->error_code;
return true;
}

diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_features.c b/tools/testing/selftests/kvm/x86_64/hyperv_features.c
index 05b32e550a80..2b6d455acf8a 100644
--- a/tools/testing/selftests/kvm/x86_64/hyperv_features.c
+++ b/tools/testing/selftests/kvm/x86_64/hyperv_features.c
@@ -18,6 +18,7 @@
static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address,
vm_vaddr_t output_address, uint64_t *hv_status)
{
+ uint64_t error_code;
uint8_t vector;

/* Note both the hypercall and the "asm safe" clobber r9-r11. */
@@ -25,7 +26,7 @@ static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address,
KVM_ASM_SAFE("vmcall")
: "=a" (*hv_status),
"+c" (control), "+d" (input_address),
- KVM_ASM_SAFE_OUTPUTS(vector)
+ KVM_ASM_SAFE_OUTPUTS(vector, error_code)
: [output_address] "r"(output_address),
"a" (-EFAULT)
: "cc", "memory", "r8", KVM_ASM_SAFE_CLOBBERS);
--
2.38.1.273.g43a17bfeac-goog