kvm: handle KVM_EXIT_MEMORY_FAULT

Upon an KVM_EXIT_MEMORY_FAULT exit, userspace needs to do the memory
conversion on the RAMBlock to turn the memory into desired attribute,
switching between private and shared.

Currently only KVM_MEMORY_EXIT_FLAG_PRIVATE in flags is valid when
KVM_EXIT_MEMORY_FAULT happens.

Note, KVM_EXIT_MEMORY_FAULT makes sense only when the RAMBlock has
guest_memfd memory backend.

Note, KVM_EXIT_MEMORY_FAULT returns with -EFAULT, so special handling is
added.

When page is converted from shared to private, the original shared
memory can be discarded via ram_block_discard_range(). Note, shared
memory can be discarded only when it's not back'ed by hugetlb because
hugetlb is supposed to be pre-allocated and no need for discarding.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>

Message-ID: <20240320083945.991426-13-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -2900,6 +2900,69 @@ static void kvm_eat_signals(CPUState *cpu)
     } while (sigismember(&chkset, SIG_IPI));
 }
 
+int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
+{
+    MemoryRegionSection section;
+    ram_addr_t offset;
+    MemoryRegion *mr;
+    RAMBlock *rb;
+    void *addr;
+    int ret = -1;
+
+    trace_kvm_convert_memory(start, size, to_private ? "shared_to_private" : "private_to_shared");
+
+    if (!QEMU_PTR_IS_ALIGNED(start, qemu_real_host_page_size()) ||
+        !QEMU_PTR_IS_ALIGNED(size, qemu_real_host_page_size())) {
+        return -1;
+    }
+
+    if (!size) {
+        return -1;
+    }
+
+    section = memory_region_find(get_system_memory(), start, size);
+    mr = section.mr;
+    if (!mr) {
+        return -1;
+    }
+
+    if (!memory_region_has_guest_memfd(mr)) {
+        error_report("Converting non guest_memfd backed memory region "
+                     "(0x%"HWADDR_PRIx" ,+ 0x%"HWADDR_PRIx") to %s",
+                     start, size, to_private ? "private" : "shared");
+        goto out_unref;
+    }
+
+    if (to_private) {
+        ret = kvm_set_memory_attributes_private(start, size);
+    } else {
+        ret = kvm_set_memory_attributes_shared(start, size);
+    }
+    if (ret) {
+        goto out_unref;
+    }
+
+    addr = memory_region_get_ram_ptr(mr) + section.offset_within_region;
+    rb = qemu_ram_block_from_host(addr, false, &offset);
+
+    if (to_private) {
+        if (rb->page_size != qemu_real_host_page_size()) {
+            /*
+             * shared memory is backed by hugetlb, which is supposed to be
+             * pre-allocated and doesn't need to be discarded
+             */
+            goto out_unref;
+        }
+        ret = ram_block_discard_range(rb, offset, size);
+    } else {
+        ret = ram_block_discard_guest_memfd_range(rb, offset, size);
+    }
+
+out_unref:
+    memory_region_unref(mr);
+    return ret;
+}
+
 int kvm_cpu_exec(CPUState *cpu)
 {
     struct kvm_run *run = cpu->kvm_run;
@@ -2967,18 +3030,20 @@ int kvm_cpu_exec(CPUState *cpu)
                 ret = EXCP_INTERRUPT;
                 break;
             }
-            fprintf(stderr, "error: kvm run failed %s\n",
+            if (!(run_ret == -EFAULT && run->exit_reason == KVM_EXIT_MEMORY_FAULT)) {
-                    strerror(-run_ret));
+                fprintf(stderr, "error: kvm run failed %s\n",
+                        strerror(-run_ret));
 #ifdef TARGET_PPC
-            if (run_ret == -EBUSY) {
+                if (run_ret == -EBUSY) {
-                fprintf(stderr,
+                    fprintf(stderr,
-                        "This is probably because your SMT is enabled.\n"
+                            "This is probably because your SMT is enabled.\n"
-                        "VCPU can only run on primary threads with all "
+                            "VCPU can only run on primary threads with all "
-                        "secondary threads offline.\n");
+                            "secondary threads offline.\n");
-            }
+                }
 #endif
-            ret = -1;
+                ret = -1;
-            break;
+                break;
+            }
         }
 
         trace_kvm_run_exit(cpu->cpu_index, run->exit_reason);
@@ -3061,6 +3126,19 @@ int kvm_cpu_exec(CPUState *cpu)
                 break;
             }
             break;
+        case KVM_EXIT_MEMORY_FAULT:
+            trace_kvm_memory_fault(run->memory_fault.gpa,
+                                   run->memory_fault.size,
+                                   run->memory_fault.flags);
+            if (run->memory_fault.flags & ~KVM_MEMORY_EXIT_FLAG_PRIVATE) {
+                error_report("KVM_EXIT_MEMORY_FAULT: Unknown flag 0x%" PRIx64,
+                             (uint64_t)run->memory_fault.flags);
+                ret = -1;
+                break;
+            }
+            ret = kvm_convert_memory(run->memory_fault.gpa, run->memory_fault.size,
+                                     run->memory_fault.flags & KVM_MEMORY_EXIT_FLAG_PRIVATE);
+            break;
         default:
             ret = kvm_arch_handle_exit(cpu, run);
             break;

accel/kvm/trace-events

@@ -31,3 +31,5 @@ kvm_cpu_exec(void) ""
 kvm_interrupt_exit_request(void) ""
 kvm_io_window_exit(void) ""
 kvm_run_exit_system_event(int cpu_index, uint32_t event_type) "cpu_index %d, system_even_type %"PRIu32
+kvm_convert_memory(uint64_t start, uint64_t size, const char *msg) "start 0x%" PRIx64 " size 0x%" PRIx64 " %s"
+kvm_memory_fault(uint64_t start, uint64_t size, uint64_t flags) "start 0x%" PRIx64 " size 0x%" PRIx64 " flags 0x%" PRIx64

include/sysemu/kvm.h

@@ -542,4 +542,6 @@ int kvm_create_guest_memfd(uint64_t size, uint64_t flags, Error **errp);
 int kvm_set_memory_attributes_private(hwaddr start, uint64_t size);
 int kvm_set_memory_attributes_shared(hwaddr start, uint64_t size);
 
+int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private);
+
 #endif