Re: Questions about the real mode in kvm/qemu

[Avi Kivity <avi.kivity@gmail.com>]

On 9/26/19 12:18 PM, Paolo Bonzini wrote:
> On 26/09/19 10:59, Maxim Levitsky wrote:
>> If you mean to ask if there is a way to let guest access use no
>> paging at all, that is access host physical addresses directly, then
>> indeed there is no way, since regular non 'unrestricted guest' mode
>> required both protected mode and paging, and 'unrestricted guest'
>> requires EPT. Academically speaking it is of course possible to
>> create paging tables that are 1:1...
> Not so academically, it's exactly what KVM does.  However, indeed it
> would also be possible to switch out of EPT mode when CR0.PG=0.  I'm not
> sure why it was done this way, maybe when the code was written it was
> simpler to use the identity map.
>
> Let's see if Avi is listening... :)


I think it was just simpler for the people who implemented it at the 
time. Switching out of EPT would have been a better solution as it would 
no longer require allocating guest physical address space with the few 
warts that requires.