Re: [PATCH 2/2] KVM: selftests: Test forced instruction emulation in dirty log test (x86 only)

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 15, 2024, Oliver Upton wrote:
> On Thu, Feb 15, 2024 at 10:50:20AM -0800, Sean Christopherson wrote:
> > Yeah, the funky flow I concocted was done purely to have the "no emulation" path
> > fall through to the common "*mem = val".  I don't have a strong preference, I
> > mentally flipped a coin on doing that versus what you suggested, and apparently
> > chose poorly :-)
> 
> Oh, I could definitely tell this was intentional :) But really if folks
> are going to add more flavors of emulated instructions to the x86
> implementation (which they should) then it might make sense to just have
> an x86-specific function.

Yeah, best prepare for the onslaught.  And if I base this on the SEV selftests
series that adds kvm_util_arch.h, it's easy to shove the x86 sequence into a
common location outside of dirty_log_test.c.  Then there are no #ifdefs or x86
code in dirty_log_test.c, and other tests can use the helper at will.

It'll require some macro hell to support all four sizes, but that's not hard,
just annoying.

And it's a good excuse to do what I should have done in the first place, and
make is_forced_emulation_enabled be available to all guest code without needing
to manually check it in each test.

Over 2-3 patches...

---
 tools/testing/selftests/kvm/dirty_log_test.c  |  9 ++++++---
 .../selftests/kvm/include/kvm_util_base.h     |  3 +++
 .../kvm/include/x86_64/kvm_util_arch.h        | 20 +++++++++++++++++++
 .../selftests/kvm/lib/x86_64/processor.c      |  3 +++
 .../selftests/kvm/x86_64/pmu_counters_test.c  |  3 ---
 .../kvm/x86_64/userspace_msr_exit_test.c      |  9 ++-------
 6 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index babea97b31a4..93c3a51a6d9b 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -114,11 +114,14 @@ static void guest_code(void)
 
 	while (true) {
 		for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {
+			uint64_t rand = READ_ONCE(random_array[i]);
+			uint64_t val = READ_ONCE(iteration);
+
 			addr = guest_test_virt_mem;
-			addr += (READ_ONCE(random_array[i]) % guest_num_pages)
-				* guest_page_size;
+			addr += (rand % guest_num_pages) * guest_page_size;
 			addr = align_down(addr, host_page_size);
-			*(uint64_t *)addr = READ_ONCE(iteration);
+
+			vcpu_arch_put_guest((u64 *)addr, val, rand);
 		}
 
 		/* Tell the host that we need more random numbers */
diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
index 4b266dc0c9bd..4b7285f073df 100644
--- a/tools/testing/selftests/kvm/include/kvm_util_base.h
+++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
@@ -610,6 +610,9 @@ void *addr_gva2hva(struct kvm_vm *vm, vm_vaddr_t gva);
 vm_paddr_t addr_hva2gpa(struct kvm_vm *vm, void *hva);
 void *addr_gpa2alias(struct kvm_vm *vm, vm_paddr_t gpa);
 
+#ifndef vcpu_arch_put_guest
+#define vcpu_arch_put_guest(mem, val, rand) do { *mem = val; } while (0)
+#endif
 
 static inline vm_paddr_t vm_untag_gpa(struct kvm_vm *vm, vm_paddr_t gpa)
 {
diff --git a/tools/testing/selftests/kvm/include/x86_64/kvm_util_arch.h b/tools/testing/selftests/kvm/include/x86_64/kvm_util_arch.h
index 205ed788aeb8..3f9a44fd4bcb 100644
--- a/tools/testing/selftests/kvm/include/x86_64/kvm_util_arch.h
+++ b/tools/testing/selftests/kvm/include/x86_64/kvm_util_arch.h
@@ -5,6 +5,8 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+extern bool is_forced_emulation_enabled;
+
 struct kvm_vm_arch {
 	uint64_t c_bit;
 	uint64_t s_bit;
@@ -20,4 +22,22 @@ static inline bool __vm_arch_has_protected_memory(struct kvm_vm_arch *arch)
 #define vm_arch_has_protected_memory(vm) \
 	__vm_arch_has_protected_memory(&(vm)->arch)
 
+/* TODO: Expand this madness to also support u8, u16, and u32 operands. */
+#define vcpu_arch_put_guest(mem, val, rand) 						\
+do {											\
+	if (!is_forced_emulation_enabled || !(rand & 1)) {				\
+		*mem = val;								\
+	} else if (rand & 2) {								\
+		__asm__ __volatile__(KVM_FEP "movq %1, %0"				\
+				     : "+m" (*mem)					\
+				     : "r" (val) : "memory");				\
+	} else {									\
+		uint64_t __old = READ_ONCE(*mem);					\
+											\
+		__asm__ __volatile__(KVM_FEP LOCK_PREFIX "cmpxchgq %[new], %[ptr]"	\
+				     : [ptr] "+m" (*mem), [old] "+a" (__old)		\
+				     : [new]"r" (val) : "memory", "cc");		\
+	}										\
+} while (0)
+
 #endif  // _TOOLS_LINUX_ASM_X86_KVM_HOST_H
diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index aa92220bf5da..d0a97d5e1ff9 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -23,6 +23,7 @@
 vm_vaddr_t exception_handlers;
 bool host_cpu_is_amd;
 bool host_cpu_is_intel;
+bool is_forced_emulation_enabled;
 
 static void regs_dump(FILE *stream, struct kvm_regs *regs, uint8_t indent)
 {
@@ -577,6 +578,7 @@ void kvm_arch_vm_post_create(struct kvm_vm *vm)
 	vm_create_irqchip(vm);
 	sync_global_to_guest(vm, host_cpu_is_intel);
 	sync_global_to_guest(vm, host_cpu_is_amd);
+	sync_global_to_guest(vm, is_forced_emulation_enabled);
 
 	if (vm->subtype == VM_SUBTYPE_SEV)
 		sev_vm_init(vm);
@@ -1337,6 +1339,7 @@ void kvm_selftest_arch_init(void)
 {
 	host_cpu_is_intel = this_cpu_is_intel();
 	host_cpu_is_amd = this_cpu_is_amd();
+	is_forced_emulation_enabled = kvm_is_forced_emulation_enabled();
 }
 
 bool sys_clocksource_is_based_on_tsc(void)
diff --git a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
index ae5f6042f1e8..6b2c1fd551b5 100644
--- a/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
+++ b/tools/testing/selftests/kvm/x86_64/pmu_counters_test.c
@@ -21,7 +21,6 @@
 
 static uint8_t kvm_pmu_version;
 static bool kvm_has_perf_caps;
-static bool is_forced_emulation_enabled;
 
 static struct kvm_vm *pmu_vm_create_with_one_vcpu(struct kvm_vcpu **vcpu,
 						  void *guest_code,
@@ -35,7 +34,6 @@ static struct kvm_vm *pmu_vm_create_with_one_vcpu(struct kvm_vcpu **vcpu,
 	vcpu_init_descriptor_tables(*vcpu);
 
 	sync_global_to_guest(vm, kvm_pmu_version);
-	sync_global_to_guest(vm, is_forced_emulation_enabled);
 
 	/*
 	 * Set PERF_CAPABILITIES before PMU version as KVM disallows enabling
@@ -609,7 +607,6 @@ int main(int argc, char *argv[])
 
 	kvm_pmu_version = kvm_cpu_property(X86_PROPERTY_PMU_VERSION);
 	kvm_has_perf_caps = kvm_cpu_has(X86_FEATURE_PDCM);
-	is_forced_emulation_enabled = kvm_is_forced_emulation_enabled();
 
 	test_intel_counters();
 
diff --git a/tools/testing/selftests/kvm/x86_64/userspace_msr_exit_test.c b/tools/testing/selftests/kvm/x86_64/userspace_msr_exit_test.c
index ab3a8c4f0b86..a409b796bb18 100644
--- a/tools/testing/selftests/kvm/x86_64/userspace_msr_exit_test.c
+++ b/tools/testing/selftests/kvm/x86_64/userspace_msr_exit_test.c
@@ -12,8 +12,6 @@
 #include "kvm_util.h"
 #include "vmx.h"
 
-static bool fep_available;
-
 #define MSR_NON_EXISTENT 0x474f4f00
 
 static u64 deny_bits = 0;
@@ -257,7 +255,7 @@ static void guest_code_filter_allow(void)
 	GUEST_ASSERT(data == 2);
 	GUEST_ASSERT(guest_exception_count == 0);
 
-	if (fep_available) {
+	if (is_forced_emulation_enabled) {
 		/* Let userspace know we aren't done. */
 		GUEST_SYNC(0);
 
@@ -519,7 +517,6 @@ static void test_msr_filter_allow(void)
 	int rc;
 
 	vm = vm_create_with_one_vcpu(&vcpu, guest_code_filter_allow);
-	sync_global_to_guest(vm, fep_available);
 
 	rc = kvm_check_cap(KVM_CAP_X86_USER_SPACE_MSR);
 	TEST_ASSERT(rc, "KVM_CAP_X86_USER_SPACE_MSR is available");
@@ -550,7 +547,7 @@ static void test_msr_filter_allow(void)
 	vcpu_run(vcpu);
 	cmd = process_ucall(vcpu);
 
-	if (fep_available) {
+	if (is_forced_emulation_enabled) {
 		TEST_ASSERT_EQ(cmd, UCALL_SYNC);
 		vm_install_exception_handler(vm, GP_VECTOR, guest_fep_gp_handler);
 
@@ -791,8 +788,6 @@ static void test_user_exit_msr_flags(void)
 
 int main(int argc, char *argv[])
 {
-	fep_available = kvm_is_forced_emulation_enabled();
-
 	test_msr_filter_allow();
 
 	test_msr_filter_deny();

base-commit: e072aa6dbd1db64323a407b3eca82dc5107ea0b1
--