Re: Questions about the real mode in kvm/qemu

[Maxim Levitsky]

On Thu, 2019-09-26 at 12:00 +0200, Paolo Bonzini wrote:
> 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
Thanks for the explanation!

Best regards,
        Maxim Levitsky