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 >
