Re: [Qemu-discuss] softmmu

[Jakob Bohm]

On 07/10/2015 09:56, MAMEESH Rania wrote:
> Does all guest memory accesses require address translation, i.e., they 
> all pass by the softmmu ?
>
> Thanks for the help.
> RM
They all require address translation (even in the case where
the addresses happen to end up the same).

However the address translation may be done by either of
3 virtual MMUs (listed from slowest to fastest).

- The softmmu in the qemu "translating" interpreter (kvm
 accelleration not used)
- The page table virtualization done by kvm (it
 reconfigures the hardware MMU to map each guest virtual
 address directly to the host physical address that would
 result from combining what the guest OS asks the virtual
 MMU to do in terms of guest virtual address to guest
 physical address mapping with what the host OS wants the
 physical MMU to do in terms of mapping guest physical
 addresses to host physical addresses).  This only does
 things when the guest OS and/or the host OS changes a
 mapping.
- The double page table hardware in some CPUs, configured
 by kvm so the guest OS controls a hardware virtualized
 MMU, which the hardware then combines with the hardware
 real MMU mapping set by the host OS.

Enjoy

Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
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