Re: [PATCH v1 10/17] virtio-mem: Paravirtualized memory hot(un)plug

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

I'm not sure if it's possible to split this up; it's a bit big.
It could also do with a pile of trace_ entries to figure out what it's
doing.


* David Hildenbrand (david@redhat.com) wrote:
> This is the very basic/initial version of virtio-mem. An introduction to
> virtio-mem can be found in the Linux kernel driver [1]. While it can be
> used in the current state for hotplug of a smaller amount of memory, it
> will heavily benefit from resizeable memory regions in the future.
> 
> Each virtio-mem device manages a memory region (provided via a memory
> backend). After requested by the hypervisor ("requested-size"), the
> guest can try to plug/unplug blocks of memory within that region, in order
> to reach the requested size. Initially, and after a reboot, all memory is
> unplugged (except in special cases - reboot during postcopy).
> 
> The guest may only try to plug/unplug blocks of memory within the usable
> region size. The usable region size is a little bigger than the
> requested size, to give the device driver some flexibility. The usable
> region size will only grow, except on reboots or when all memory is
> requested to get unplugged. The guest can never plug more memory than
> requested. Unplugged memory will get zapped/discarded, similar to in a
> balloon device.
> 
> The block size is variable, however, it is always chosen in a way such that
> THP splits are avoided (e.g., 2MB). The state of each block
> (plugged/unplugged) is tracked in a bitmap.
> 
> As virtio-mem devices (e.g., virtio-mem-pci) will be memory devices, we now
> expose "VirtioMEMDeviceInfo" via "query-memory-devices".
> 
> --------------------------------------------------------------------------
> 
> There are two important follow-up items that are in the works:
> 1. Resizeable memory regions: Use resizeable allocations/RAM blocks to
>    grow/shrink along with the usable region size. This avoids creating
>    initially very big VMAs, RAM blocks, and KVM slots.
> 2. Protection of unplugged memory: Make sure the gust cannot actually
>    make use of unplugged memory.
> 
> Other follow-up items that are in the works:
> 1. Exclude unplugged memory during migration (via precopy notifier).
> 2. Handle remapping of memory.
> 3. Support for other architectures.
> 
> --------------------------------------------------------------------------
> 
> Example usage (virtio-mem-pci is introduced in follow-up patches):
> 
> Start QEMU with two virtio-mem devices (one per NUMA node):
>  $ qemu-system-x86_64 -m 4G,maxmem=20G \
>   -smp sockets=2,cores=2 \
>   -numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \
>   [...]
>   -object memory-backend-ram,id=mem0,size=8G \
>   -device virtio-mem-pci,id=vm0,memdev=mem0,node=0,requested-size=0M \
>   -object memory-backend-ram,id=mem1,size=8G \
>   -device virtio-mem-pci,id=vm1,memdev=mem1,node=1,requested-size=1G
> 
> Query the configuration:
>  (qemu) info memory-devices
>  Memory device [virtio-mem]: "vm0"
>    memaddr: 0x140000000
>    node: 0
>    requested-size: 0
>    size: 0
>    max-size: 8589934592
>    block-size: 2097152
>    memdev: /objects/mem0
>  Memory device [virtio-mem]: "vm1"
>    memaddr: 0x340000000
>    node: 1
>    requested-size: 1073741824
>    size: 1073741824
>    max-size: 8589934592
>    block-size: 2097152
>    memdev: /objects/mem1
> 
> Add some memory to node 0:
>  (qemu) qom-set vm0 requested-size 500M
> 
> Remove some memory from node 1:
>  (qemu) qom-set vm1 requested-size 200M
> 
> Query the configuration again:
>  (qemu) info memory-devices
>  Memory device [virtio-mem]: "vm0"
>    memaddr: 0x140000000
>    node: 0
>    requested-size: 524288000
>    size: 524288000
>    max-size: 8589934592
>    block-size: 2097152
>    memdev: /objects/mem0
>  Memory device [virtio-mem]: "vm1"
>    memaddr: 0x340000000
>    node: 1
>    requested-size: 209715200
>    size: 209715200
>    max-size: 8589934592
>    block-size: 2097152
>    memdev: /objects/mem1
> 
> [1] https://lkml.kernel.org/r/20200311171422.10484-1-david@redhat.com
> 
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Markus Armbruster <armbru@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/virtio/Kconfig              |  11 +
>  hw/virtio/Makefile.objs        |   1 +
>  hw/virtio/virtio-mem.c         | 762 +++++++++++++++++++++++++++++++++
>  include/hw/virtio/virtio-mem.h |  80 ++++
>  qapi/misc.json                 |  39 +-
>  5 files changed, 892 insertions(+), 1 deletion(-)
>  create mode 100644 hw/virtio/virtio-mem.c
>  create mode 100644 include/hw/virtio/virtio-mem.h
> 
> diff --git a/hw/virtio/Kconfig b/hw/virtio/Kconfig
> index 83122424fa..0eda25c4e1 100644
> --- a/hw/virtio/Kconfig
> +++ b/hw/virtio/Kconfig
> @@ -47,3 +47,14 @@ config VIRTIO_PMEM
>      depends on VIRTIO
>      depends on VIRTIO_PMEM_SUPPORTED
>      select MEM_DEVICE
> +
> +config VIRTIO_MEM_SUPPORTED
> +    bool
> +
> +config VIRTIO_MEM
> +    bool
> +    default y
> +    depends on VIRTIO
> +    depends on LINUX
> +    depends on VIRTIO_MEM_SUPPORTED
> +    select MEM_DEVICE
> diff --git a/hw/virtio/Makefile.objs b/hw/virtio/Makefile.objs
> index 4e4d39a0a4..7df70e977e 100644
> --- a/hw/virtio/Makefile.objs
> +++ b/hw/virtio/Makefile.objs
> @@ -18,6 +18,7 @@ common-obj-$(call land,$(CONFIG_VIRTIO_PMEM),$(CONFIG_VIRTIO_PCI)) += virtio-pme
>  obj-$(call land,$(CONFIG_VHOST_USER_FS),$(CONFIG_VIRTIO_PCI)) += vhost-user-fs-pci.o
>  obj-$(CONFIG_VIRTIO_IOMMU) += virtio-iommu.o
>  obj-$(CONFIG_VHOST_VSOCK) += vhost-vsock.o
> +obj-$(CONFIG_VIRTIO_MEM) += virtio-mem.o
>  
>  ifeq ($(CONFIG_VIRTIO_PCI),y)
>  obj-$(CONFIG_VHOST_VSOCK) += vhost-vsock-pci.o
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> new file mode 100644
> index 0000000000..e25b2c74f2
> --- /dev/null
> +++ b/hw/virtio/virtio-mem.c
> @@ -0,0 +1,762 @@
> +/*
> + * Virtio MEM device
> + *
> + * Copyright (C) 2020 Red Hat, Inc.
> + *
> + * Authors:
> + *  David Hildenbrand <david@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "qemu-common.h"
> +#include "qemu/iov.h"
> +#include "qemu/cutils.h"
> +#include "qemu/error-report.h"
> +#include "qemu/units.h"
> +#include "sysemu/numa.h"
> +#include "sysemu/sysemu.h"
> +#include "sysemu/reset.h"
> +#include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-bus.h"
> +#include "hw/virtio/virtio-access.h"
> +#include "hw/virtio/virtio-mem.h"
> +#include "qapi/error.h"
> +#include "qapi/visitor.h"
> +#include "exec/ram_addr.h"
> +#include "migration/misc.h"
> +#include "migration/postcopy-ram.h"
> +#include "hw/boards.h"
> +#include "hw/qdev-properties.h"
> +#include "config-devices.h"
> +
> +/*
> + * Use QEMU_VMALLOC_ALIGN, so no THP will have to be split when unplugging
> + * memory (e.g., 2MB on x86_64).
> + */
> +#define VIRTIO_MEM_MIN_BLOCK_SIZE QEMU_VMALLOC_ALIGN
> +/*
> + * Size the usable region bigger than the requested size if possible. Esp.
> + * Linux guests will only add (aligned) memory blocks in case they fully
> + * fit into the usable region, but plug+online only a subset of the pages.
> + * The memory block size corresponds mostly to the section size.
> + *
> + * This allows e.g., to add 20MB with a section size of 128MB on x86_64, and
> + * a section size of 1GB on arm64 (as long as the start address is properly
> + * aligned, similar to ordinary DIMMs).
> + *
> + * We can change this at any time and maybe even make it configurable if
> + * necessary (as the section size can change). But it's more likely that the
> + * section size will rather get smaller and not bigger over time.
> + */
> +#if defined(__x86_64__)
> +#define VIRTIO_MEM_USABLE_EXTENT (2 * (128 * MiB))
> +#else
> +#error VIRTIO_MEM_USABLE_EXTENT not defined
> +#endif
> +
> +static bool virtio_mem_discard_inhibited(void)
> +{
> +    PostcopyState ps = postcopy_state_get();
> +
> +    /* Postcopy cannot deal with concurrent discards (yet), so it's special. */
> +    return ps >= POSTCOPY_INCOMING_DISCARD && ps < POSTCOPY_INCOMING_END;
> +}
> +
> +static bool virtio_mem_test_bitmap(VirtIOMEM *vmem, uint64_t start_gpa,
> +                                   uint64_t size, bool plug)
> +{
> +    uint64_t bit = (start_gpa - vmem->addr) / vmem->block_size;
> +
> +    g_assert(QEMU_IS_ALIGNED(start_gpa, vmem->block_size));
> +    g_assert(QEMU_IS_ALIGNED(size, vmem->block_size));
> +    g_assert(vmem->bitmap);
> +
> +    while (size) {
> +        g_assert((bit / BITS_PER_BYTE) <= vmem->bitmap_size);
> +
> +        if (plug && !test_bit(bit, vmem->bitmap)) {
> +            return false;
> +        } else if (!plug && test_bit(bit, vmem->bitmap)) {
> +            return false;
> +        }
> +        size -= vmem->block_size;
> +        bit++;
> +    }
> +    return true;
> +}
> +
> +static void virtio_mem_set_bitmap(VirtIOMEM *vmem, uint64_t start_gpa,
> +                                  uint64_t size, bool plug)
> +{
> +    const uint64_t bit = (start_gpa - vmem->addr) / vmem->block_size;
> +    const uint64_t nbits = size / vmem->block_size;
> +
> +    g_assert(QEMU_IS_ALIGNED(start_gpa, vmem->block_size));
> +    g_assert(QEMU_IS_ALIGNED(size, vmem->block_size));
> +    g_assert(vmem->bitmap);

This bit/nbits/alignment checking could be split out and shared between
these two functions.

> +    if (plug) {
> +        bitmap_set(vmem->bitmap, bit, nbits);
> +    } else {
> +        bitmap_clear(vmem->bitmap, bit, nbits);
> +    }
> +}
> +
> +static void virtio_mem_send_response(VirtIOMEM *vmem, VirtQueueElement *elem,
> +                                     struct virtio_mem_resp *resp)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(vmem);
> +    VirtQueue *vq = vmem->vq;
> +
> +    iov_from_buf(elem->in_sg, elem->in_num, 0, resp, sizeof(*resp));
> +
> +    virtqueue_push(vq, elem, sizeof(*resp));
> +    virtio_notify(vdev, vq);
> +}
> +
> +static void virtio_mem_send_response_simple(VirtIOMEM *vmem,
> +                                            VirtQueueElement *elem,
> +                                            uint16_t type)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(vmem);
> +    struct virtio_mem_resp resp = {};
> +
> +    virtio_stw_p(vdev, &resp.type, type);
> +    virtio_mem_send_response(vmem, elem, &resp);
> +}
> +
> +static void virtio_mem_bad_request(VirtIOMEM *vmem, const char *msg)
> +{
> +    virtio_error(VIRTIO_DEVICE(vmem), "virtio-mem protocol violation: %s", msg);
> +}
> +
> +static bool virtio_mem_valid_range(VirtIOMEM *vmem, uint64_t gpa, uint64_t size)
> +{
> +    if (!QEMU_IS_ALIGNED(gpa, vmem->block_size)) {
> +            return false;
> +    }
> +    if (gpa + size < gpa || size == 0) {
> +        return false;
> +    }
> +    if (gpa < vmem->addr || gpa >= vmem->addr + vmem->usable_region_size) {
> +        return false;
> +    }
> +    if (gpa + size > vmem->addr + vmem->usable_region_size) {
> +        return false;
> +    }
> +    return true;
> +}
> +
> +static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa,
> +                                      uint64_t size, bool plug)
> +{
> +    const uint64_t offset = start_gpa - vmem->addr;
> +    int ret;
> +
> +    if (!plug) {
> +        if (virtio_mem_discard_inhibited()) {
> +            return -EBUSY;
> +        }
> +        /* Note: Discarding should never fail at this point. */
> +        ret = ram_block_discard_range(vmem->memdev->mr.ram_block, offset, size);
> +        if (ret) {

error_report ?

> +            return -EBUSY;
> +        }
> +    }
> +    virtio_mem_set_bitmap(vmem, start_gpa, size, plug);
> +    return 0;
> +}
> +
> +static int virtio_mem_state_change_request(VirtIOMEM *vmem, uint64_t gpa,
> +                                           uint16_t nb_blocks, bool plug)
> +{
> +    const uint64_t size = nb_blocks * vmem->block_size;
> +    int ret;
> +
> +    if (!virtio_mem_valid_range(vmem, gpa, size)) {
> +        return VIRTIO_MEM_RESP_ERROR;
> +    }
> +
> +    if (plug && (vmem->size + size > vmem->requested_size)) {
> +        return VIRTIO_MEM_RESP_NACK;
> +    }
> +
> +    /* test if really all blocks are in the opposite state */
> +    if (!virtio_mem_test_bitmap(vmem, gpa, size, !plug)) {
> +        return VIRTIO_MEM_RESP_ERROR;
> +    }
> +
> +    ret = virtio_mem_set_block_state(vmem, gpa, size, plug);
> +    if (ret) {
> +        return VIRTIO_MEM_RESP_BUSY;
> +    }
> +    if (plug) {
> +        vmem->size += size;
> +    } else {
> +        vmem->size -= size;
> +    }
> +    return VIRTIO_MEM_RESP_ACK;
> +}
> +
> +static void virtio_mem_plug_request(VirtIOMEM *vmem, VirtQueueElement *elem,
> +                                    struct virtio_mem_req *req)
> +{
> +    const uint64_t gpa = le64_to_cpu(req->u.plug.addr);
> +    const uint16_t nb_blocks = le16_to_cpu(req->u.plug.nb_blocks);
> +    uint16_t type;
> +
> +    type = virtio_mem_state_change_request(vmem, gpa, nb_blocks, true);
> +    virtio_mem_send_response_simple(vmem, elem, type);
> +}
> +
> +static void virtio_mem_unplug_request(VirtIOMEM *vmem, VirtQueueElement *elem,
> +                                      struct virtio_mem_req *req)
> +{
> +    const uint64_t gpa = le64_to_cpu(req->u.unplug.addr);
> +    const uint16_t nb_blocks = le16_to_cpu(req->u.unplug.nb_blocks);
> +    uint16_t type;
> +
> +    type = virtio_mem_state_change_request(vmem, gpa, nb_blocks, false);
> +    virtio_mem_send_response_simple(vmem, elem, type);
> +}
> +
> +static void virtio_mem_resize_usable_region(VirtIOMEM *vmem,
> +                                            uint64_t requested_size,
> +                                            bool can_shrink)
> +{
> +    uint64_t newsize = MIN(memory_region_size(&vmem->memdev->mr),
> +                           requested_size + VIRTIO_MEM_USABLE_EXTENT);
> +
> +    /* We must only grow while the guest is running. */
> +    if (newsize < vmem->usable_region_size && !can_shrink) {
> +        return;
> +    }
> +
> +    vmem->usable_region_size = newsize;
> +}
> +
> +static int virtio_mem_unplug_all(VirtIOMEM *vmem)
> +{
> +    RAMBlock *rb = vmem->memdev->mr.ram_block;
> +    int ret;
> +
> +    if (virtio_mem_discard_inhibited()) {
> +        return -EBUSY;
> +    }
> +
> +    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
> +    if (ret) {
> +        /* Note: Discarding should never fail at this point. */

error_report?

> +        return -EBUSY;
> +    }
> +    bitmap_clear(vmem->bitmap, 0, vmem->bitmap_size);
> +    vmem->size = 0;
> +
> +    virtio_mem_resize_usable_region(vmem, vmem->requested_size, true);
> +    return 0;
> +}
> +
> +static void virtio_mem_unplug_all_request(VirtIOMEM *vmem,
> +                                          VirtQueueElement *elem)
> +{
> +
> +    if (virtio_mem_unplug_all(vmem)) {
> +        virtio_mem_send_response_simple(vmem, elem, VIRTIO_MEM_RESP_BUSY);
> +    } else {
> +        virtio_mem_send_response_simple(vmem, elem, VIRTIO_MEM_RESP_ACK);
> +    }
> +}
> +
> +static void virtio_mem_state_request(VirtIOMEM *vmem, VirtQueueElement *elem,
> +                                     struct virtio_mem_req *req)
> +{
> +    const uint64_t gpa = le64_to_cpu(req->u.state.addr);
> +    const uint16_t nb_blocks = le16_to_cpu(req->u.state.nb_blocks);
> +    const uint64_t size = nb_blocks * vmem->block_size;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(vmem);
> +    struct virtio_mem_resp resp = {};
> +
> +    if (!virtio_mem_valid_range(vmem, gpa, size)) {
> +        virtio_mem_send_response_simple(vmem, elem, VIRTIO_MEM_RESP_ERROR);
> +        return;
> +    }
> +
> +    virtio_stw_p(vdev, &resp.type, VIRTIO_MEM_RESP_ACK);
> +    if (virtio_mem_test_bitmap(vmem, gpa, size, true)) {
> +        virtio_stw_p(vdev, &resp.u.state.state, VIRTIO_MEM_STATE_PLUGGED);
> +    } else if (virtio_mem_test_bitmap(vmem, gpa, size, false)) {
> +        virtio_stw_p(vdev, &resp.u.state.state, VIRTIO_MEM_STATE_UNPLUGGED);
> +    } else {
> +        virtio_stw_p(vdev, &resp.u.state.state, VIRTIO_MEM_STATE_MIXED);
> +    }
> +    virtio_mem_send_response(vmem, elem, &resp);
> +}
> +
> +static void virtio_mem_handle_request(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    const int len = sizeof(struct virtio_mem_req);
> +    VirtIOMEM *vmem = VIRTIO_MEM(vdev);
> +    VirtQueueElement *elem;
> +    struct virtio_mem_req req;
> +    uint64_t type;
> +
> +    while (true) {
> +        elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> +        if (!elem) {
> +            return;
> +        }
> +
> +        if (iov_to_buf(elem->out_sg, elem->out_num, 0, &req, len) < len) {
> +            virtio_mem_bad_request(vmem, "invalid request size");

Print the size.

> +            g_free(elem);
> +            return;
> +        }
> +
> +        if (iov_size(elem->in_sg, elem->in_num) <
> +            sizeof(struct virtio_mem_resp)) {
> +            virtio_mem_bad_request(vmem, "not enough space for response");
> +            g_free(elem);
> +            return;
> +        }
> +
> +        type = le16_to_cpu(req.type);
> +        switch (type) {
> +        case VIRTIO_MEM_REQ_PLUG:
> +            virtio_mem_plug_request(vmem, elem, &req);
> +            break;
> +        case VIRTIO_MEM_REQ_UNPLUG:
> +            virtio_mem_unplug_request(vmem, elem, &req);
> +            break;
> +        case VIRTIO_MEM_REQ_UNPLUG_ALL:
> +            virtio_mem_unplug_all_request(vmem, elem);
> +            break;
> +        case VIRTIO_MEM_REQ_STATE:
> +            virtio_mem_state_request(vmem, elem, &req);
> +            break;
> +        default:
> +            virtio_mem_bad_request(vmem, "unknown request type");

Could include the type .


> +            g_free(elem);
> +            return;
> +        }
> +
> +        g_free(elem);
> +    }
> +}
> +
> +static void virtio_mem_get_config(VirtIODevice *vdev, uint8_t *config_data)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(vdev);
> +    struct virtio_mem_config *config = (void *) config_data;
> +
> +    config->block_size = cpu_to_le32(vmem->block_size);
> +    config->node_id = cpu_to_le16(vmem->node);
> +    config->requested_size = cpu_to_le64(vmem->requested_size);
> +    config->plugged_size = cpu_to_le64(vmem->size);
> +    config->addr = cpu_to_le64(vmem->addr);
> +    config->region_size = cpu_to_le64(memory_region_size(&vmem->memdev->mr));
> +    config->usable_region_size = cpu_to_le64(vmem->usable_region_size);
> +}
> +
> +static uint64_t virtio_mem_get_features(VirtIODevice *vdev, uint64_t features,
> +                                        Error **errp)
> +{
> +    MachineState *ms = MACHINE(qdev_get_machine());
> +
> +    if (ms->numa_state) {
> +#if defined(CONFIG_ACPI)
> +        virtio_add_feature(&features, VIRTIO_MEM_F_ACPI_PXM);
> +#endif
> +    }
> +    return features;
> +}
> +
> +static void virtio_mem_system_reset(void *opaque)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(opaque);
> +
> +    /*
> +     * During usual resets, we will unplug all memory and shrink the usable
> +     * region size. This is, however, not possible in all scenarios. Then,
> +     * the guest has to deal with this manually (VIRTIO_MEM_REQ_UNPLUG_ALL).
> +     */
> +    virtio_mem_unplug_all(vmem);
> +}
> +
> +static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
> +{
> +    MachineState *ms = MACHINE(qdev_get_machine());
> +    int nb_numa_nodes = ms->numa_state ? ms->numa_state->num_nodes : 0;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VirtIOMEM *vmem = VIRTIO_MEM(dev);
> +    uint64_t page_size;
> +    RAMBlock *rb;
> +    int ret;
> +
> +    if (!vmem->memdev) {
> +        error_setg(errp, "'%s' property must be set", VIRTIO_MEM_MEMDEV_PROP);
> +        return;
> +    } else if (host_memory_backend_is_mapped(vmem->memdev)) {
> +        char *path = object_get_canonical_path_component(OBJECT(vmem->memdev));
> +
> +        error_setg(errp, "can't use already busy memdev: %s", path);
> +        g_free(path);
> +        return;
> +    }
> +
> +    if ((nb_numa_nodes && vmem->node >= nb_numa_nodes) ||
> +        (!nb_numa_nodes && vmem->node)) {
> +        error_setg(errp, "Property '%s' has value '%" PRIu32
> +                   "', which exceeds the number of numa nodes: %d",
> +                   VIRTIO_MEM_NODE_PROP, vmem->node,
> +                   nb_numa_nodes ? nb_numa_nodes : 1);
> +        return;
> +    }
> +
> +    if (enable_mlock) {
> +        error_setg(errp, "not compatible with mlock yet");
> +        return;
> +    }
> +
> +    if (!memory_region_is_ram(&vmem->memdev->mr) ||
> +        memory_region_is_rom(&vmem->memdev->mr) ||
> +        !vmem->memdev->mr.ram_block) {
> +        error_setg(errp, "unsupported memdev");
> +        return;
> +    }
> +
> +    rb = vmem->memdev->mr.ram_block;
> +    page_size = qemu_ram_pagesize(rb);
> +
> +    if (vmem->block_size < page_size) {
> +        error_setg(errp, "'%s' has to be at least the page size (0x%"
> +                   PRIx64 ")", VIRTIO_MEM_BLOCK_SIZE_PROP, page_size);
> +        return;
> +    } else if (!QEMU_IS_ALIGNED(vmem->requested_size, vmem->block_size)) {
> +        error_setg(errp, "'%s' has to be multiples of '%s' (0x%" PRIx32
> +                   ")", VIRTIO_MEM_REQUESTED_SIZE_PROP,
> +                   VIRTIO_MEM_BLOCK_SIZE_PROP, vmem->block_size);
> +        return;
> +    } else if (!QEMU_IS_ALIGNED(memory_region_size(&vmem->memdev->mr),
> +                                vmem->block_size)) {
> +        error_setg(errp, "'%s' backend size has to be multiples of '%s' (0x%"
> +                   PRIx32 ")", VIRTIO_MEM_MEMDEV_PROP,
> +                   VIRTIO_MEM_BLOCK_SIZE_PROP, vmem->block_size);
> +        return;
> +    }
> +
> +    if (ram_block_discard_set_required(true)) {
> +        error_setg(errp, "Discarding RAM is marked broken.");
> +        return;
> +    }
> +
> +    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
> +    if (ret) {
> +        /* Note: Discarding should never fail at this point. */
> +        error_setg_errno(errp, -ret, "Discarding RAM failed.");
> +        ram_block_discard_set_required(false);
> +        return;
> +    }
> +
> +    virtio_mem_resize_usable_region(vmem, vmem->requested_size, true);
> +
> +    vmem->bitmap_size = memory_region_size(&vmem->memdev->mr) /
> +                        vmem->block_size;
> +    vmem->bitmap = bitmap_new(vmem->bitmap_size);
> +
> +    virtio_init(vdev, TYPE_VIRTIO_MEM, VIRTIO_ID_MEM,
> +                sizeof(struct virtio_mem_config));
> +    vmem->vq = virtio_add_queue(vdev, 128, virtio_mem_handle_request);
> +
> +    host_memory_backend_set_mapped(vmem->memdev, true);
> +    vmstate_register_ram(&vmem->memdev->mr, DEVICE(vmem));
> +    qemu_register_reset(virtio_mem_system_reset, vmem);
> +    return;
> +}
> +
> +static void virtio_mem_device_unrealize(DeviceState *dev, Error **errp)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VirtIOMEM *vmem = VIRTIO_MEM(dev);
> +
> +    qemu_unregister_reset(virtio_mem_system_reset, vmem);
> +    vmstate_unregister_ram(&vmem->memdev->mr, DEVICE(vmem));
> +    host_memory_backend_set_mapped(vmem->memdev, false);
> +    virtio_del_queue(vdev, 0);
> +    virtio_cleanup(vdev);
> +    g_free(vmem->bitmap);
> +    ramblock_discard_set_required(false);
> +}
> +
> +static int virtio_mem_pre_save(void *opaque)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(opaque);
> +
> +    vmem->migration_addr = vmem->addr;
> +    vmem->migration_block_size = vmem->block_size;

You might look at VMSTATE_WITH_TMP could avoid you having the dummy
fields.

> +    return 0;
> +}
> +
> +static int virtio_mem_restore_unplugged(VirtIOMEM *vmem)
> +{
> +    unsigned long bit;
> +    uint64_t offset;
> +    int ret;
> +
> +    /* TODO: Better postcopy handling - defer to postcopy end. */
> +    if (virtio_mem_discard_inhibited()) {
> +        return 0;
> +    }
> +
> +    bit = find_first_zero_bit(vmem->bitmap, vmem->bitmap_size);
> +    while (bit < vmem->bitmap_size) {
> +        offset = bit * vmem->block_size;
> +
> +        if (offset + vmem->block_size >=
> +            memory_region_size(&vmem->memdev->mr)) {
> +            break;
> +        }
> +        /* Note: Discarding should never fail at this point. */
> +        ret = ram_block_discard_range(vmem->memdev->mr.ram_block, offset,
> +                                      vmem->block_size);
> +        if (ret) {
> +            return -EINVAL;
> +        }
> +        bit = find_next_zero_bit(vmem->bitmap, vmem->bitmap_size, bit + 1);
> +    }
> +    return 0;
> +}
> +
> +static int virtio_mem_post_load(void *opaque, int version_id)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(opaque);
> +
> +    if (vmem->migration_block_size != vmem->block_size) {
> +        error_report("'%s' doesn't match", VIRTIO_MEM_BLOCK_SIZE_PROP);
> +        return -EINVAL;
> +    }
> +    if (vmem->migration_addr != vmem->addr) {
> +        error_report("'%s' doesn't match", VIRTIO_MEM_ADDR_PROP);
> +        return -EINVAL;
> +    }
> +    return virtio_mem_restore_unplugged(vmem);
> +}
> +
> +static const VMStateDescription vmstate_virtio_mem_device = {
> +    .name = "virtio-mem-device",
> +    .minimum_version_id = 1,
> +    .version_id = 1,
> +    .pre_save = virtio_mem_pre_save,
> +    .post_load = virtio_mem_post_load,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_UINT64(usable_region_size, VirtIOMEM),
> +        VMSTATE_UINT64(size, VirtIOMEM),
> +        VMSTATE_UINT64(requested_size, VirtIOMEM),
> +        VMSTATE_UINT64(migration_addr, VirtIOMEM),
> +        VMSTATE_UINT32(migration_block_size, VirtIOMEM),
> +        VMSTATE_BITMAP(bitmap, VirtIOMEM, 0, bitmap_size),
> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
> +static const VMStateDescription vmstate_virtio_mem = {
> +    .name = "virtio-mem",
> +    .minimum_version_id = 1,
> +    .version_id = 1,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_VIRTIO_DEVICE,
> +        VMSTATE_END_OF_LIST()
> +    },
> +};
> +
> +static void virtio_mem_fill_device_info(const VirtIOMEM *vmem,
> +                                        VirtioMEMDeviceInfo *vi)
> +{
> +    vi->memaddr = vmem->addr;
> +    vi->node = vmem->node;
> +    vi->requested_size = vmem->requested_size;
> +    vi->size = vmem->size;
> +    vi->max_size = memory_region_size(&vmem->memdev->mr);
> +    vi->block_size = vmem->block_size;
> +    vi->memdev = object_get_canonical_path(OBJECT(vmem->memdev));
> +}
> +
> +static MemoryRegion *virtio_mem_get_memory_region(VirtIOMEM *vmem, Error **errp)
> +{
> +    if (!vmem->memdev) {
> +        error_setg(errp, "'%s' property must be set", VIRTIO_MEM_MEMDEV_PROP);
> +        return NULL;
> +    }
> +
> +    return &vmem->memdev->mr;
> +}
> +
> +static void virtio_mem_get_size(Object *obj, Visitor *v, const char *name,
> +                                void *opaque, Error **errp)
> +{
> +    const VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +    uint64_t value = vmem->size;
> +
> +    visit_type_size(v, name, &value, errp);
> +}
> +
> +static void virtio_mem_get_requested_size(Object *obj, Visitor *v,
> +                                          const char *name, void *opaque,
> +                                          Error **errp)
> +{
> +    const VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +    uint64_t value = vmem->requested_size;
> +
> +    visit_type_size(v, name, &value, errp);
> +}
> +
> +static void virtio_mem_set_requested_size(Object *obj, Visitor *v,
> +                                          const char *name, void *opaque,
> +                                          Error **errp)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +    Error *err = NULL;
> +    uint64_t value;
> +
> +    visit_type_size(v, name, &value, &err);
> +    if (err) {
> +        error_propagate(errp, err);
> +        return;
> +    }
> +
> +    /*
> +     * The block size and memory backend are not fixed until the device was
> +     * realized. realize() will verify these properties then.
> +     */
> +    if (DEVICE(obj)->realized) {
> +        if (!QEMU_IS_ALIGNED(value, vmem->block_size)) {
> +            error_setg(errp, "'%s' has to be multiples of '%s' (0x%" PRIx32
> +                       ")", name, VIRTIO_MEM_BLOCK_SIZE_PROP,
> +                       vmem->block_size);
> +            return;
> +        } else if (value > memory_region_size(&vmem->memdev->mr)) {
> +            error_setg(errp, "'%s' cannot exceed the memory backend size"
> +                       "(0x%" PRIx64 ")", name,
> +                       memory_region_size(&vmem->memdev->mr));
> +            return;
> +        }
> +
> +        if (value != vmem->requested_size) {
> +            virtio_mem_resize_usable_region(vmem, value, false);
> +            vmem->requested_size = value;
> +        }
> +        /*
> +         * Trigger a config update so the guest gets notified. We trigger
> +         * even if the size didn't change (especially helpful for debugging).
> +         */
> +        virtio_notify_config(VIRTIO_DEVICE(vmem));
> +    } else {
> +        vmem->requested_size = value;
> +    }
> +}
> +
> +static void virtio_mem_get_block_size(Object *obj, Visitor *v, const char *name,
> +                                      void *opaque, Error **errp)
> +{
> +    const VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +    uint64_t value = vmem->block_size;
> +
> +    visit_type_size(v, name, &value, errp);
> +}
> +
> +static void virtio_mem_set_block_size(Object *obj, Visitor *v, const char *name,
> +                                      void *opaque, Error **errp)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +    Error *err = NULL;
> +    uint64_t value;
> +
> +    if (DEVICE(obj)->realized) {
> +        error_setg(errp, "'%s' cannot be changed", name);
> +        return;
> +    }
> +
> +    visit_type_size(v, name, &value, &err);
> +    if (err) {
> +        error_propagate(errp, err);
> +        return;
> +    }
> +
> +    if (value > UINT32_MAX) {
> +        error_setg(errp, "'%s' has to be smaller than 0x%" PRIx32, name,
> +                   UINT32_MAX);
> +        return;
> +    } else if (value < VIRTIO_MEM_MIN_BLOCK_SIZE) {
> +        error_setg(errp, "'%s' has to be at least 0x%" PRIx32, name,
> +                   VIRTIO_MEM_MIN_BLOCK_SIZE);
> +        return;
> +    } else if (!is_power_of_2(value)) {
> +        error_setg(errp, "'%s' has to be a power of two", name);
> +        return;
> +    }
> +    vmem->block_size = value;
> +}
> +
> +static void virtio_mem_instance_init(Object *obj)
> +{
> +    VirtIOMEM *vmem = VIRTIO_MEM(obj);
> +
> +    vmem->block_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
> +
> +    object_property_add(obj, VIRTIO_MEM_SIZE_PROP, "size", virtio_mem_get_size,
> +                        NULL, NULL, NULL, &error_abort);
> +    object_property_add(obj, VIRTIO_MEM_REQUESTED_SIZE_PROP, "size",
> +                        virtio_mem_get_requested_size,
> +                        virtio_mem_set_requested_size, NULL, NULL,
> +                        &error_abort);
> +    object_property_add(obj, VIRTIO_MEM_BLOCK_SIZE_PROP, "size",
> +                        virtio_mem_get_block_size, virtio_mem_set_block_size,
> +                        NULL, NULL, &error_abort);
> +}
> +
> +static Property virtio_mem_properties[] = {
> +    DEFINE_PROP_UINT64(VIRTIO_MEM_ADDR_PROP, VirtIOMEM, addr, 0),
> +    DEFINE_PROP_UINT32(VIRTIO_MEM_NODE_PROP, VirtIOMEM, node, 0),
> +    DEFINE_PROP_LINK(VIRTIO_MEM_MEMDEV_PROP, VirtIOMEM, memdev,
> +                     TYPE_MEMORY_BACKEND, HostMemoryBackend *),
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
> +static void virtio_mem_class_init(ObjectClass *klass, void *data)
> +{
> +    DeviceClass *dc = DEVICE_CLASS(klass);
> +    VirtioDeviceClass *vdc = VIRTIO_DEVICE_CLASS(klass);
> +    VirtIOMEMClass *vmc = VIRTIO_MEM_CLASS(klass);
> +
> +    device_class_set_props(dc, virtio_mem_properties);
> +    dc->vmsd = &vmstate_virtio_mem;
> +
> +    set_bit(DEVICE_CATEGORY_MISC, dc->categories);
> +    vdc->realize = virtio_mem_device_realize;
> +    vdc->unrealize = virtio_mem_device_unrealize;
> +    vdc->get_config = virtio_mem_get_config;
> +    vdc->get_features = virtio_mem_get_features;
> +    vdc->vmsd = &vmstate_virtio_mem_device;
> +
> +    vmc->fill_device_info = virtio_mem_fill_device_info;
> +    vmc->get_memory_region = virtio_mem_get_memory_region;
> +}
> +
> +static const TypeInfo virtio_mem_info = {
> +    .name = TYPE_VIRTIO_MEM,
> +    .parent = TYPE_VIRTIO_DEVICE,
> +    .instance_size = sizeof(VirtIOMEM),
> +    .instance_init = virtio_mem_instance_init,
> +    .class_init = virtio_mem_class_init,
> +    .class_size = sizeof(VirtIOMEMClass),
> +};
> +
> +static void virtio_register_types(void)
> +{
> +    type_register_static(&virtio_mem_info);
> +}
> +
> +type_init(virtio_register_types)
> diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
> new file mode 100644
> index 0000000000..27158cb611
> --- /dev/null
> +++ b/include/hw/virtio/virtio-mem.h
> @@ -0,0 +1,80 @@
> +/*
> + * Virtio MEM device
> + *
> + * Copyright (C) 2020 Red Hat, Inc.
> + *
> + * Authors:
> + *  David Hildenbrand <david@redhat.com>
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#ifndef HW_VIRTIO_MEM_H
> +#define HW_VIRTIO_MEM_H
> +
> +#include "standard-headers/linux/virtio_mem.h"
> +#include "hw/virtio/virtio.h"
> +#include "qapi/qapi-types-misc.h"
> +#include "sysemu/hostmem.h"
> +
> +#define TYPE_VIRTIO_MEM "virtio-mem"
> +
> +#define VIRTIO_MEM(obj) \
> +        OBJECT_CHECK(VirtIOMEM, (obj), TYPE_VIRTIO_MEM)
> +#define VIRTIO_MEM_CLASS(oc) \
> +        OBJECT_CLASS_CHECK(VirtIOMEMClass, (oc), TYPE_VIRTIO_MEM)
> +#define VIRTIO_MEM_GET_CLASS(obj) \
> +        OBJECT_GET_CLASS(VirtIOMEMClass, (obj), TYPE_VIRTIO_MEM)
> +
> +#define VIRTIO_MEM_MEMDEV_PROP "memdev"
> +#define VIRTIO_MEM_NODE_PROP "node"
> +#define VIRTIO_MEM_SIZE_PROP "size"
> +#define VIRTIO_MEM_REQUESTED_SIZE_PROP "requested-size"
> +#define VIRTIO_MEM_BLOCK_SIZE_PROP "block-size"
> +#define VIRTIO_MEM_ADDR_PROP "memaddr"
> +
> +typedef struct VirtIOMEM {
> +    VirtIODevice parent_obj;
> +
> +    /* guest -> host request queue */
> +    VirtQueue *vq;
> +
> +    /* bitmap used to track unplugged memory */
> +    int32_t bitmap_size;
> +    unsigned long *bitmap;
> +
> +    /* assigned memory backend and memory region */
> +    HostMemoryBackend *memdev;
> +
> +    /* NUMA node */
> +    uint32_t node;
> +
> +    /* assigned address of the region in guest physical memory */
> +    uint64_t addr;
> +    uint64_t migration_addr;
> +
> +    /* usable region size (<= region_size) */
> +    uint64_t usable_region_size;
> +
> +    /* actual size (how much the guest plugged) */
> +    uint64_t size;
> +
> +    /* requested size */
> +    uint64_t requested_size;
> +
> +    /* block size and alignment */
> +    uint32_t block_size;
> +    uint32_t migration_block_size;
> +} VirtIOMEM;
> +
> +typedef struct VirtIOMEMClass {
> +    /* private */
> +    VirtIODevice parent;
> +
> +    /* public */
> +    void (*fill_device_info)(const VirtIOMEM *vmen, VirtioMEMDeviceInfo *vi);
> +    MemoryRegion *(*get_memory_region)(VirtIOMEM *vmem, Error **errp);
> +} VirtIOMEMClass;
> +
> +#endif
> diff --git a/qapi/misc.json b/qapi/misc.json
> index 99b90ac80b..feaeacec22 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -1354,19 +1354,56 @@
>            }
>  }
>  
> +##
> +# @VirtioMEMDeviceInfo:
> +#
> +# VirtioMEMDevice state information
> +#
> +# @id: device's ID
> +#
> +# @memaddr: physical address in memory, where device is mapped
> +#
> +# @requested-size: the user requested size of the device
> +#
> +# @size: the (current) size of memory that the device provides
> +#
> +# @max-size: the maximum size of memory that the device can provide
> +#
> +# @block-size: the block size of memory that the device provides
> +#
> +# @node: NUMA node number where device is assigned to
> +#
> +# @memdev: memory backend linked with the region
> +#
> +# Since: 5.1
> +##
> +{ 'struct': 'VirtioMEMDeviceInfo',
> +  'data': { '*id': 'str',
> +            'memaddr': 'size',
> +            'requested-size': 'size',
> +            'size': 'size',
> +            'max-size': 'size',
> +            'block-size': 'size',
> +            'node': 'int',
> +            'memdev': 'str'
> +          }
> +}
> +
>  ##
>  # @MemoryDeviceInfo:
>  #
>  # Union containing information about a memory device
>  #
>  # nvdimm is included since 2.12. virtio-pmem is included since 4.1.
> +# virtio-mem is included since 5.2.
>  #
>  # Since: 2.1
>  ##
>  { 'union': 'MemoryDeviceInfo',
>    'data': { 'dimm': 'PCDIMMDeviceInfo',
>              'nvdimm': 'PCDIMMDeviceInfo',
> -            'virtio-pmem': 'VirtioPMEMDeviceInfo'
> +            'virtio-pmem': 'VirtioPMEMDeviceInfo',
> +            'virtio-mem': 'VirtioMEMDeviceInfo'
>            }
>  }
>  
> -- 
> 2.25.3
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK