Re: Questions about the real mode in kvm/qemu

[Paolo Bonzini]

On 26/09/19 11:41, Maxim Levitsky wrote:
>> Thinking more about it, I suppose that saves memory (the same EPT page
>> tables can now be used independent of guest CR0.PG), at the cost of
>> making TLB misses a little slower.
> Don't really understand what you mean. 
> Isn't this always the case that EPT and guest paging
> are independent (at least when no nesting is involved)?

There are two possibilities:

1) emulate CR0.PG=0 with EPT + identity page

- advantage: the EPT pages will be reused once the guest sets CR0.PG=1

- disadvantage: TLB misses have to walk two levels of page tables

2) emulate CR0.PG=0 with EPT disabled.  Similar to ept=0, CR3 will point
to PAE page tables that do the HVA->GPA transition.

- advantage: faster TLB misses

- disadvantage: need to build separate page tables for CR0.PG=1 (EPT
format) and CR0.PG=0 (PAE format), need to "waste" 4k of GPA space for
the identity map

Paolo