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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 522
SEC. 7.7
I/O VIRTUALIZATION
491
hardware devices was one of the reasons VM/370 became popular: companies
wanted to buy new and faster hardware but did not want to change their software.
Virtual machine technology made this possible.
Another interesting trend related to I/O is that the hypervisor can take the role
of a virtual switch. In this case, each virtual machine has a MAC address and the
hypevisor switches frames from one virtual machine to another—just like an Ether-
net switch would do. Virtual switches have several advantages. For instance, it is
very easy to reconfigure them. Also, it is possible to augment the switch with addi-
tional functionality, for instance for additional security.
I/O MMUs
Another I/O problem that must be solved somehow is the use of DMA, which
uses absolute memory addresses.
As might be expected, the hypervisor has to
intervene here and remap the addresses before the DMA starts.
However, hard-
ware already exists with an
I/O MMU
, which virtualizes the I/O the same way the
MMU virtualizes the memory.
I/O MMU exists in different forms and shapes for
many processor architectures. Even if we limit ourselves to the x86, Intel and
AMD have slightly different technology. Still, the idea is the same. This hardware
eliminates the DMA problem.
Just like regular MMUs, the I/O MMU uses page tables to map a memory ad-
dress that a device wants to use (the device address) to a physical address. In a vir-
tual environment, the hypervisor can set up the page tables in such a way that a de-
vice performing DMA will not trample over memory that does not belong to the
virtual machine on whose behalf it is working.
I/O MMUs offer different advantages when dealing with a device in a virtu-
alized world.
Device pass through
allows the physical device to be directly as-
signed to a particular virtual machine. In general, it would be ideal if device ad-
dress space were exactly the same as the guest’s physical address space. However,
this is unlikely—unless you have an I/O MMU. The MMU allows the addresses to
remapped transparently, and both the device and the virtual machine are blissfully
unaware of the address translation that takes place under the hood.
Device isolation
ensures that a device assigned to a virtual machine can direct-
ly access that virtual machine without jeopardizing the integrity of the other guests.
In other words, the I/O MMU prevents rogue DMA traffic, just as a normal MMU
prevents rogue memory accesses from processes—in both cases accesses to
unmapped pages result in faults.
DMA and addresses are not the whole I/O story, unfortunately.
For complete-
ness, we also need to virtualize interrupts, so that the interrupt generated by a de-
vice arrives at the right virtual machine, with the right interrupt number. Modern
I/O MMUs therefore support
interrupt remapping
.
Say, a device sends a mes-
sage signaled interrupt with number 1. This message first hits the I/O MMU that
will use the interrupt remapping table to translate to a new message destined for
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