Re: Questions about the real mode in kvm/qemu

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/09/19 11:24, Maxim Levitsky wrote:
> On Thu, 2019-09-26 at 11:18 +0200, 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.
> You mean KVM uses 1:1 EPT pages and no guest paging,
> to allow guest to access host physical address space?

No, it uses the usual HVA->GPA EPT pages and 1:1 GPA->GVA pages when EPT
is enabled and guest CR0.PG=0.  This lets KVM work around the CR0.PG=1
requirement when unrestricted guest mode.

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.

Thanks,

Paolo

>>   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... :)
>>
>> Paolo
> 
> Here a quote from the PRM:
> 
> "The first processors to support VMX operation require CR0.PE and CR0.PG to be 1 in VMX operation (see Section
> 23.8). This restriction implies that guest software cannot be run in unpaged protected mode or in real-address
> mode. Later processors support a VM-execution control called “unrestricted guest”. 1 If this control is 1, CR0.PE and
> CR0.PG may be 0 in VMX non-root operation. Such processors allow guest software to run in unpaged protected
> mode or in real-address mode. The following items describe the behavior of such software:"
> ...
> 
> "As noted in Section 26.2.1.1, the “enable EPT” VM-execution control must be 1 if the “unrestricted guest” VM-execution control is 1."
> 
> 
> Best regards,
> 	Maxim Levitsky
>