Hi Maxim, Thanks much for your reply. On Sun, Oct 11, 2020 at 3:29 AM Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote: > > On Sun, 2020-10-11 at 01:26 -0400, harry harry wrote: > > Hi QEMU/KVM developers, > > > > I am sorry if my email disturbs you. I did an experiment and found the > > guest physical addresses (GPAs) are not the same as the corresponding > > host virtual addresses (HVAs). I am curious about why; I think they > > should be the same. I am very appreciated if you can give some > > comments and suggestions about 1) why GPAs and HVAs are not the same > > in the following experiment; 2) are there any better experiments to > > look into the reasons? Any other comments/suggestions are also very > > welcome. Thanks! > > > > The experiment is like this: in a single vCPU VM, I ran a program > > allocating and referencing lots of pages (e.g., 100*1024) and didn't > > let the program terminate. Then, I checked the program's guest virtual > > addresses (GVAs) and GPAs through parsing its pagemap and maps files > > located at /proc/pid/pagemap and /proc/pid/maps, respectively. At > > last, in the host OS, I checked the vCPU's pagemap and maps files to > > find the program's HVAs and host physical addresses (HPAs); I actually > > checked the new allocated physical pages in the host OS after the > > program was executed in the guest OS. > > > > With the above experiment, I found GPAs of the program are different > > from its corresponding HVAs. BTW, Intel EPT and other related Intel > > virtualization techniques were enabled. > > > > Thanks, > > Harry > > > The fundemental reason is that some HVAs (e.g. QEMU's virtual memory addresses) are already allocated > for qemu's own use (e.g qemu code/heap/etc) prior to the guest starting up. > > KVM does though use quite effiecient way of mapping HVA's to GPA. It uses an array of arbitrary sized HVA areas > (which we call memslots) and for each such area/memslot you specify the GPA to map to. In theory QEMU > could allocate the whole guest's memory in one contiguous area and map it as single memslot to the guest. > In practice there are MMIO holes, and various other reasons why there will be more that 1 memslot. It is still not clear to me why GPAs are not the same as the corresponding HVAs in my experiment. Since two-dimensional paging (Intel EPT) is used, GPAs should be the same as their corresponding HVAs. Otherwise, I think EPT may not work correctly. What do you think? Thanks, Harry
