What is "reserved memory" in linux

There's Hyper-V VPS with 512 MB of memory

root@vps:~$ dmesg | grep Memory:
[    0.000000] Memory: 478420k/524144k available (7764k kernel code, 392k absent, 45332k reserved, 5967k data, 1980k init)

and free (like /proc/meminfo) shows only 486M (524144k - 478420k ?)

root@uk:~$ free -h
              total        used        free      shared  buff/cache   available
Mem:           413M         97M         79M        4.6M        236M        299M
Swap:            0B          0B          0B

part of dmesg

root@ru:~$ dmesg | head -n 50
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.10.0-1062.18.1.el7.x86_64 (mockbuild@kbuilder.bsys.centos.org) (gcc version 4.8.5 20150623 (Red H
at 4.8.5-39) (GCC) ) #1 SMP Tue Mar 17 23:49:17 UTC 2020
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.10.0-1062.18.1.el7.x86_64 root=UUID=ad1530fe-f286-4adf-b8ad-d0bf6232f031
 ro crashkernel=auto rhgb quiet LANG=en_US.UTF-8
[    0.000000] e820: BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000001ffeffff] usable
[    0.000000] BIOS-e820: [mem 0x000000001fff0000-0x000000001fffefff] ACPI data
[    0.000000] BIOS-e820: [mem 0x000000001ffff000-0x000000001fffffff] ACPI NVS
[    0.000000] NX (Execute Disable) protection: active
[    0.000000] SMBIOS 2.3 present.
[    0.000000] DMI: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090006  04/28/2016
[    0.000000] Hypervisor detected: Microsoft HyperV
[    0.000000] HyperV: features 0x2e7f, hints 0xc2c
[    0.000000] Hyper-V Host Build:14393-10.0-1-0.1884
[    0.000000] HyperV: LAPIC Timer Frequency: 0x30d40
[    0.000000] tsc: Marking TSC unstable due to running on Hyper-V
[    0.000000] Hyper-V: Using ext hypercall for remote TLB flush
[    0.000000] e820: update [mem 0x00000000-0x00000fff] usable ==> reserved
[    0.000000] e820: remove [mem 0x000a0000-0x000fffff] usable
[    0.000000] e820: last_pfn = 0x1fff0 max_arch_pfn = 0x400000000
[    0.000000] MTRR default type: uncachable
[    0.000000] MTRR fixed ranges enabled:
[    0.000000]   00000-9FFFF write-back
[    0.000000]   A0000-DFFFF uncachable
[    0.000000]   E0000-FFFFF write-back
[    0.000000] MTRR variable ranges enabled:
[    0.000000]   0 base 00000000000 mask FFF00000000 write-back
[    0.000000]   1 disabled
[    0.000000]   2 disabled
[    0.000000]   3 disabled
[    0.000000]   4 disabled
[    0.000000]   5 disabled
[    0.000000]   6 disabled
[    0.000000]   7 disabled
[    0.000000] PAT configuration [0-7]: WB  WC  UC- UC  WB  WP  UC- UC
[    0.000000] found SMP MP-table at [mem 0x000ff780-0x000ff78f] mapped at [ffffffffff200780]
[    0.000000] Base memory trampoline at [ffff8de5c0099000] 99000 size 24576
[    0.000000] Using GB pages for direct mapping
[    0.000000] BRK [0x0bc70000, 0x0bc70fff] PGTABLE
[    0.000000] BRK [0x0bc71000, 0x0bc71fff] PGTABLE
[    0.000000] BRK [0x0bc72000, 0x0bc72fff] PGTABLE
[    0.000000] BRK [0x0bc73000, 0x0bc73fff] PGTABLE
[    0.000000] BRK [0x0bc74000, 0x0bc74fff] PGTABLE
[    0.000000] RAMDISK: [mem 0x1e05e000-0x1f14cfff]
[    0.000000] Early table checksum verification disabled
[    0.000000] ACPI: RSDP 00000000000f56f0 00014 (v00 ACPIAM)

cat /proc/iomem

00000000-00000fff : reserved
00001000-0009fbff : System RAM
0009fc00-0009ffff : reserved
000a0000-000bffff : PCI Bus 0000:00
000c0000-000c7fff : Video ROM
000e0000-000fffff : reserved
  000f0000-000fffff : System ROM
00100000-1ffeffff : System RAM
  0aa00000-0b19525e : Kernel code
  0b19525f-0b76923f : Kernel data
  0b962000-0bc6efff : Kernel bss
1fff0000-1fffefff : ACPI Tables
1ffff000-1fffffff : ACPI Non-volatile Storage
f8000000-fffbffff : PCI Bus 0000:00
  f8000000-fbffffff : 0000:00:08.0
    f8000000-f87fffff : 5620e0c7-8062-4dce-aeb7-520c7ef76171
  fec00000-fec003ff : IOAPIC 0
  fee00000-fee00fff : Local APIC
    fee00000-fee00fff : pnp 00:07
fffc0000-ffffffff : pnp 00:08
fe0000000-fffffffff : PCI Bus 0000:00

Distro is Centos 7, if matters. Where that "reserved" memory goes and its it possible to.. take it back? And same thing in KVM VM, but much better - from it's reserved "only" precious 47mb (from 512)..