[Bug 226673] real memory > physical memory (amd64 Virtualbox guest)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=226673

            Bug ID: 226673
           Summary: real memory > physical memory (amd64 Virtualbox guest)
           Product: Base System
           Version: 11.1-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: freebsd at oldach.net

For some reason, an 11-STABLE amd64 system running under VirtualBox reports
more physical memory than allocated by the host. For example, an allocation of
4096 MB physical memory by VirtualBox results in:

FreeBSD 11.1-STABLE #0 r330790: Fri Mar 16 20:51:59 CET 2018
    root at latitude.local:/usr/obj/amd64.amd64/usr/src/sys/GENERIC amd64
...
real memory  = 4831838208 (4608 MB)
avail memory = 4104138752 (3914 MB)

sysctl also reports an odd value for hw.realmem:

hw.physmem: 4260958208
hw.usermem: 4167311360
hw.realmem: 536870912

All figures are OK when running an i386 kernel in the same VM (but of course
lower than in amd64 mode):

FreeBSD 11.1-STABLE #0 r330790: Mon Mar 12 08:56:08 CET 2018
    toor at latitude.local:/usr/obj/usr/src/sys/HMO i386
...
real memory  = 3758030848 (3583 MB)
avail memory = 3678859264 (3508 MB)

hw.physmem: 3744882688
hw.usermem: 3710955520
hw.realmem: 3758030848

And also when reducing memory allocation from 4096 MB to 3072 MB the amd64
figures are sensible:

FreeBSD 11.1-STABLE #0 r330790: Fri Mar 16 20:51:59 CET 2018
    root at latitude.local:/usr/obj/amd64.amd64/usr/src/sys/GENERIC amd64
...
real memory  = 3221159936 (3071 MB)
avail memory = 3079446528 (2936 MB)

hw.physmem: 3187220480
hw.usermem: 3106312192
hw.realmem: 3221159936

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