[PATCH 10/13] KVM: selftests: Add macro to atomically sync per-VM "global" pointers

[Sean Christopherson]

Add atomic_sync_global_pointer_to_guest() to allow sync'ing "global"
pointers that hold per-VM values, i.e. technically need to be handled in
a thread-safe manner.

Use the new macro to fix a mostly-theoretical bug where ARM's ucall MMIO
setup could result in different VMs stomping on each other.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 .../testing/selftests/kvm/include/kvm_util_base.h | 15 +++++++++++++++
 tools/testing/selftests/kvm/lib/aarch64/ucall.c   | 15 +++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
index 8ce9e5be70a3..f4a2622db53c 100644
--- a/tools/testing/selftests/kvm/include/kvm_util_base.h
+++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
@@ -16,6 +16,7 @@
 #include <linux/kvm.h>
 #include "linux/rbtree.h"
 
+#include <asm/atomic.h>
 
 #include <sys/ioctl.h>
 
@@ -727,6 +728,20 @@ kvm_userspace_memory_region_find(struct kvm_vm *vm, uint64_t start,
 	memcpy(&(g), _p, sizeof(g));				\
 })
 
+/*
+ * Sync a global pointer to the guest that has a per-VM value, in which case
+ * writes to the host copy of the "global" must be serialized (in case a test
+ * is being truly crazy and spawning multiple VMs concurrently).
+ */
+#define atomic_sync_global_pointer_to_guest(vm, g, val) ({	\
+	typeof(g) *_p = addr_gva2hva(vm, (vm_vaddr_t)&(g));	\
+								\
+	while (cmpxchg(&g, NULL, val))				\
+		;						\
+	memcpy(_p, &(g), sizeof(g));				\
+	WRITE_ONCE(g, NULL);					\
+})
+
 void assert_on_unhandled_exception(struct kvm_vcpu *vcpu);
 
 void vcpu_arch_dump(FILE *stream, struct kvm_vcpu *vcpu,
diff --git a/tools/testing/selftests/kvm/lib/aarch64/ucall.c b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
index 132c0e98bf49..c30a6eacde34 100644
--- a/tools/testing/selftests/kvm/lib/aarch64/ucall.c
+++ b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
@@ -6,8 +6,17 @@
  */
 #include "kvm_util.h"
 
+/*
+ * This "global" holds different per-VM values, it must not be accessed from
+ * host code except to sync the guest value, and that must be done atomically.
+ */
 static vm_vaddr_t *ucall_exit_mmio_addr;
 
+static void ucall_set_mmio_addr(struct kvm_vm *vm, vm_vaddr_t *val)
+{
+	atomic_sync_global_pointer_to_guest(vm, ucall_exit_mmio_addr, val);
+}
+
 static bool ucall_mmio_init(struct kvm_vm *vm, vm_paddr_t gpa)
 {
 	if (kvm_userspace_memory_region_find(vm, gpa, gpa + 1))
@@ -15,8 +24,7 @@ static bool ucall_mmio_init(struct kvm_vm *vm, vm_paddr_t gpa)
 
 	virt_pg_map(vm, gpa, gpa);
 
-	ucall_exit_mmio_addr = (vm_vaddr_t *)gpa;
-	sync_global_to_guest(vm, ucall_exit_mmio_addr);
+	ucall_set_mmio_addr(vm, (vm_vaddr_t *)gpa);
 
 	return true;
 }
@@ -66,8 +74,7 @@ void ucall_arch_init(struct kvm_vm *vm, void *arg)
 
 void ucall_arch_uninit(struct kvm_vm *vm)
 {
-	ucall_exit_mmio_addr = 0;
-	sync_global_to_guest(vm, ucall_exit_mmio_addr);
+	ucall_set_mmio_addr(vm, (vm_vaddr_t *)NULL);
 }
 
 void ucall_arch_do_ucall(vm_vaddr_t uc)
-- 
2.37.1.595.g718a3a8f04-goog


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