For testing, it can be useful to simulate an enormous amount of memory
(e.g. 2^64 RAM). This adds an MMIO device that acts as sparse memory.
When something writes a nonzero value to a sparse-mem address, we
allocate a block of memory. This block is kept around, until all of the
bytes within the block are zero-ed. The device has a very low priority
(so it can be mapped beneath actual RAM, and virtual device MMIO
regions).
Signed-off-by: Alexander Bulekov <alx...@bu.edu>
---
MAINTAINERS | 1 +
hw/mem/meson.build | 1 +
hw/mem/sparse-mem.c | 154 ++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 156 insertions(+)
create mode 100644 hw/mem/sparse-mem.c
diff --git a/MAINTAINERS b/MAINTAINERS
index f22d83c178..9e3d8b1401 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2618,6 +2618,7 @@ R: Thomas Huth <th...@redhat.com>
S: Maintained
F: tests/qtest/fuzz/
F: scripts/oss-fuzz/
+F: hw/mem/sparse-mem.c
F: docs/devel/fuzzing.rst
Register API
diff --git a/hw/mem/meson.build b/hw/mem/meson.build
index 0d22f2b572..732f459e0a 100644
--- a/hw/mem/meson.build
+++ b/hw/mem/meson.build
@@ -1,5 +1,6 @@
mem_ss = ss.source_set()
mem_ss.add(files('memory-device.c'))
+mem_ss.add(files('sparse-mem.c'))
mem_ss.add(when: 'CONFIG_DIMM', if_true: files('pc-dimm.c'))
mem_ss.add(when: 'CONFIG_NPCM7XX', if_true: files('npcm7xx_mc.c'))
mem_ss.add(when: 'CONFIG_NVDIMM', if_true: files('nvdimm.c'))
diff --git a/hw/mem/sparse-mem.c b/hw/mem/sparse-mem.c
new file mode 100644
index 0000000000..ffda6f76b4
--- /dev/null
+++ b/hw/mem/sparse-mem.c
@@ -0,0 +1,154 @@
+/*
+ * A sparse memory device
+ *
+ * Copyright Red Hat Inc., 2021
+ *
+ * Authors:
+ * Alexander Bulekov <alx...@bu.edu>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+
+#include "exec/address-spaces.h"
+#include "hw/qdev-properties.h"
+
+#define TYPE_SPARSE_MEM "sparse-mem"
+#define SPARSE_MEM(obj) OBJECT_CHECK(SparseMemState, (obj), TYPE_SPARSE_MEM)
+
+#define SPARSE_BLOCK_SIZE 0x1000
+
+typedef struct SparseMemState {
+ DeviceState parent_obj;
+ MemoryRegion mmio;
+ uint64_t baseaddr;
+ uint64_t length;
+ uint64_t usage;
+ uint64_t maxsize;
+ GHashTable *mapped;
+} SparseMemState;
+
+typedef struct sparse_mem_block {
+ uint16_t nonzeros;
+ uint8_t data[SPARSE_BLOCK_SIZE];
+} sparse_mem_block;
+
+static uint64_t sparse_mem_read(void *opaque, hwaddr addr, unsigned int size)
+{
+ SparseMemState *s = opaque;
+ uint64_t ret = 0;
+ size_t pfn = addr / SPARSE_BLOCK_SIZE;
+ size_t offset = addr % SPARSE_BLOCK_SIZE;
+ sparse_mem_block *block;
+
+ block = g_hash_table_lookup(s->mapped, (void *)pfn);
+ if (block) {
+ assert(offset + size <= sizeof(block->data));
+ memcpy(&ret, block->data + offset, size);
+ }
+ return ret;
+}
+
+static void sparse_mem_write(void *opaque, hwaddr addr, uint64_t v,
+ unsigned int size)
+{
+ SparseMemState *s = opaque;
+ size_t pfn = addr / SPARSE_BLOCK_SIZE;
+ size_t offset = addr % SPARSE_BLOCK_SIZE;
+ int nonzeros = 0;
+ sparse_mem_block *block;
+
+ if (!g_hash_table_lookup(s->mapped, (void *)pfn) &&
+ s->usage + SPARSE_BLOCK_SIZE < s->maxsize && v) {
+ g_hash_table_insert(s->mapped, (void *)pfn,
+ g_new0(sparse_mem_block, 1));
+ s->usage += sizeof(block->data);
+ }
+ block = g_hash_table_lookup(s->mapped, (void *)pfn);
+ if (!block) {
+ return;
+ }
+
+ assert(offset + size <= sizeof(block->data));
+
+ /*
+ * Track the number of nonzeros, so we can adjust the block's nonzero count
+ * after writing the value v
+ */
+ for (int i = 0; i < size; i++) {
+ nonzeros -= (block->data[offset + i] != 0);
+ }
+
+ memcpy(block->data + offset, &v, size);
+
+ for (int i = 0; i < size; i++) {
+ nonzeros += (block->data[offset + i] != 0);
+ }
+
+ /* Update the number of nonzeros in the block, free it, if it's empty */
+ assert(block->nonzeros + nonzeros < sizeof(block->data));
+ assert((int)block->nonzeros + nonzeros >= 0);
+ block->nonzeros += nonzeros;
+
+ if (block->nonzeros == 0) {
+ g_free(block);
+ g_hash_table_remove(s->mapped, (void *)pfn);
+ s->usage -= sizeof(block->data);
+ }
+}
+
+static const MemoryRegionOps sparse_mem_ops = {
+ .read = sparse_mem_read,
+ .write = sparse_mem_write,
+ .endianness = DEVICE_LITTLE_ENDIAN,
+ .valid = {
+ .min_access_size = 1,
+ .max_access_size = 8,
+ .unaligned = false,
+ },
+};
+
+static Property sparse_mem_properties[] = {
+ /* The base address of the memory */
+ DEFINE_PROP_UINT64("baseaddr", SparseMemState, baseaddr, 0x0),
+ /* The length of the sparse memory region */
+ DEFINE_PROP_UINT64("length", SparseMemState, length, UINT64_MAX),
+ /* Max amount of actual memory that can be used to back the sparse memory
*/
+ DEFINE_PROP_UINT64("maxsize", SparseMemState, maxsize, 0x100000),
+ DEFINE_PROP_END_OF_LIST(),
+};
+
+static void sparse_mem_realize(DeviceState *dev, Error **errp)
+{
+ SparseMemState *s = SPARSE_MEM(dev);
+
+ assert(s->baseaddr + s->length > s->baseaddr);
+
+ s->mapped = g_hash_table_new(NULL, NULL);
+ memory_region_init_io(&(s->mmio), OBJECT(s), &sparse_mem_ops, s,
+ "sparse-mem", s->length);
+ memory_region_add_subregion_overlap(get_system_memory(), s->baseaddr,
+ &(s->mmio), -100);
+}
+
+static void sparse_mem_class_init(ObjectClass *klass, void *data)
+{
+ DeviceClass *dc = DEVICE_CLASS(klass);
+
+ device_class_set_props(dc, sparse_mem_properties);
+
+ dc->desc = "Sparse Memory Device";
+ dc->realize = sparse_mem_realize;
+}
+
+static const TypeInfo sparse_mem_types[] = {
+ {
+ .name = TYPE_SPARSE_MEM,
+ .parent = TYPE_DEVICE,
+ .instance_size = sizeof(SparseMemState),
+ .class_init = sparse_mem_class_init,
+ },
+};
+DEFINE_TYPES(sparse_mem_types);
--
2.28.0
