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

[David Hildenbrand]

On 03.07.20 11:18, David Hildenbrand wrote:
> 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).

#if defined(TARGET_X86_64) || defined(TARGET_I386)

to be precise.


-- 
Thanks,

David / dhildenb