Re: Why guest physical addresses are not the same as the corresponding host virtual addresses in QEMU/KVM? Thanks!

Hi Sean, Thanks for the prompt reply. Please see my following reply. Thanks. On Tue, Oct 13, 2020 at 12:52 AM Sean Christopherson < sean.j.christopherson(a)intel.com&gt; wrote: the host KVM is a with HVAs. i.e. GPAs, to intents and purely software when look like: then does the different Let me give you one concrete example as follows to explain the meaning of ``different''. Suppose a program is running in a single-vCPU VM. The program allocates and references one page (e.g., array[1024*4]). Assume that allocating and referencing the page in the guest OS triggers a page fault and host OS allocates a machine page to back it. Assume that GVA of array[0] is 0x000000000021 and its corresponding GPA is 0x0000000000000081. I think array[0]'s corresponding HVA should also be 0x0000000000000081, which is the same as array[0]'s GPA. If array[0]'s HVA is not 0x0000000000000081, array[0]'s GPA is* different* from its corresponding HVA. Now, let's assume array[0]'s GPA is different from its corresponding HVA. I think there might be one issue like this: I think MMU's hardware logic to translate ``GPA ->[extended/nested page tables] -> HPA''[1] should be the same as ``VA-> [page tables] -> PA"[2]; if true, how does KVM find the correct HPA with the different HVA (e.g., array[0]'s HVA is not 0x0000000000000081) when there are EPT violations? [1] Please note that this hardware walk is the last step, which only translates the guest physical address to the host physical address through the four-level nested page table. [2] Please note that this hardware walk assumes translating the VA to the PA without virtualization involvement. Please note that the above addresses are not real and just use for explanations. Thanks, Harry