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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 517
486
VIRTUALIZATION AND THE CLOUD
CHAP. 7
7.6 MEMORY VIRTUALIZATION
So far we have addressed the issue of how to virtualize the CPU.
But a com-
puter system has more than just a CPU.
It also has memory and I/O devices. They
have to be virtualized, too.
Let us see how that is done.
Modern operating systems nearly all support virtual memory, which is basical-
ly a mapping of pages in the virtual address space onto pages of physical memory.
This mapping is defined by (multilevel) page tables.
Typically the mapping is set
in motion by having the operating system set a control register in the CPU that
points to the top-level page table.
Virtualization greatly complicates memory man-
agement. In fact, it took hardware manufacturers two tries to get it right.
Suppose, for example, a virtual machine is running, and the guest operating
system in it decides to map its virtual pages 7, 4, and 3 onto physical pages 10, 11,
and 12, respectively. It builds page tables containing this mapping and loads a
hardware register to point to the top-level page table.
This instruction is sensitive.
On a VT CPU, it will trap; with dynamic translation it will cause a call to a hyper-
visor procedure; on a paravirtualized operating system, it will generate a hypercall.
For simplicity, let us assume it traps into a type 1 hypervisor, but the problem is the
same in all three cases.
What does the hypervisor do now? One solution is to actually allocate physi-
cal pages 10, 11, and 12 to this virtual machine and set up the actual page tables to
map the virtual machine’s virtual pages 7, 4, and 3 to use them.
So far, so good.
Now suppose a second virtual machine starts and maps its virtual pages 4, 5,
and 6 onto physical pages 10, 11, and 12 and loads the control register to point to
its page tables.
The hypervisor catches the trap, but what should it do?
It cannot
use this mapping because physical pages 10, 11, and 12 are already in use.
It can
find some free pages, say 20, 21, and 22, and use them, but it first has to create new
page tables mapping the virtual pages 4, 5, and 6 of virtual machine 2 onto 20, 21,
and 22.
If another virtual machine starts and tries to use physical pages 10, 11, and
12, it has to create a mapping for them.
In general, for each virtual machine the
hypervisor needs to create a
shadow page table
that maps the virtual pages used
by the virtual machine onto the actual pages the hypervisor gave it.
Worse yet, every time the guest operating system changes its page tables, the
hypervisor must change the shadow page tables as well.
For example, if the guest
OS remaps virtual page 7 onto what it sees as physical page 200 (instead of 10),
the hypervisor has to know about this change.
The trouble is that the guest operat-
ing system can change its page tables by just writing to memory. No sensitive oper-
ations are required, so the hypervisor does not even know about the change and
certainly cannot update the shadow page tables used by the actual hardware.
A possible (but clumsy) solution is for the hypervisor to keep track of which
page in the guest’s virtual memory contains the top-level page table. It can get this
information the first time the guest attempts to load the hardware register that
points to it because this instruction is sensitive and traps. The hypervisor can create
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