setting of intel EPT Accessed and dirty FLags for guest page tables

Question

I am reading Intel virtualization manual where manual says that if bit 6 of EPTP(a VM execution control field) is set, the processor will set the Accessed and dirty bits in relevant EPT entries according to some rules.

I am trying to understand that if processor sets A/D bits in EPT on access and modification of relevant pages how guest Operating will get benefit from this setting as guest Os has no access to EPT. In my understanding A/D bits are used by memory manager of the OS for optimization and swapping algorithms and there is no role of these bits in page walker.

I(being programmer of VMM) have to add code in VMM to search the relevant entry in GPA space and mark the bits accordingly? If this is the case then how can we say that these bits are set with out the knowledge of VMM? kvm way of dealing this issue will be a good answer also

Answer 1

In general, the guest OS would not benefit from the access and dirty bits in the EPT from being set. As you stated the guest does not typically have access to the EPT. This is purely for the hypervisor/VMM. It is analogous to the dirty and access bit in a process page table, the process does not use it, only the OS.

With regard to your second question, it is a bit unclear so I'm not sure what you are asking. However, the hardware will mark the access and dirty bits assuming it has been set up correctly, you do not have to do it manually.