Re: [PULL 13/41] virtio-mem: Paravirtualized memory hot(un)plug

[David Hildenbrand]

On 03.07.20 11:04, Michael S. Tsirkin wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> 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>
> Message-Id: <20200626072248.78761-11-david@redhat.com>
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  qapi/misc.json                 |  39 +-
>  include/hw/virtio/virtio-mem.h |  78 ++++
>  hw/virtio/virtio-mem.c         | 724 +++++++++++++++++++++++++++++++++
>  hw/virtio/Kconfig              |  11 +
>  hw/virtio/Makefile.objs        |   1 +
>  5 files changed, 852 insertions(+), 1 deletion(-)
>  create mode 100644 include/hw/virtio/virtio-mem.h
>  create mode 100644 hw/virtio/virtio-mem.c
> 
> diff --git a/qapi/misc.json b/qapi/misc.json
> index a5a0beb902..65ca3edf32 100644
> --- a/qapi/misc.json
> +++ b/qapi/misc.json
> @@ -1356,19 +1356,56 @@
>            }
>  }
>  
> +##
> +# @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 5.1.
>  #
>  # Since: 2.1
>  ##
>  { 'union': 'MemoryDeviceInfo',
>    'data': { 'dimm': 'PCDIMMDeviceInfo',
>              'nvdimm': 'PCDIMMDeviceInfo',
> -            'virtio-pmem': 'VirtioPMEMDeviceInfo'
> +            'virtio-pmem': 'VirtioPMEMDeviceInfo',
> +            'virtio-mem': 'VirtioMEMDeviceInfo'
>            }
>  }
>  
> diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h
> new file mode 100644
> index 0000000000..6981096f7c
> --- /dev/null
> +++ b/include/hw/virtio/virtio-mem.h
> @@ -0,0 +1,78 @@
> +/*
> + * 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;
> +
> +    /* 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 */
> +    uint64_t 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/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> new file mode 100644
> index 0000000000..d8a0c974d3
> --- /dev/null
> +++ b/hw/virtio/virtio-mem.c
> @@ -0,0 +1,724 @@
> +/*
> + * 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 "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))

I just did a cross-compile on s390x and noticed that this should be
guarded by defined(TARGET_X86_64) (it's target dependent).

Sorry for the noise.

-- 
Thanks,

David / dhildenb