On 10/07/20 19:13, Jim Mattson wrote: > On Fri, Jul 10, 2020 at 10:06 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: >> >> On 10/07/20 18:30, Jim Mattson wrote: >>>> >>>> This can be problem when having a mixed setup of machines with 5-level page >>>> tables and machines with 4-level page tables, as live migration can change >>>> MAXPHYADDR while the guest runs, which can theoretically introduce bugs. >>> >>> Huh? Changing MAXPHYADDR while the guest runs should be illegal. Or >>> have I missed some peculiarity of LA57 that makes MAXPHYADDR a dynamic >>> CPUID information field? >> >> Changing _host_ MAXPHYADDR while the guest runs, such as if you migrate >> from a host-maxphyaddr==46 to a host-maxphyaddr==52 machine (while >> keeping guest-maxphyaddr==46). > > Ah, but what does that have to do with LA57? Intel only has MAXPHYADDR > 46 on LA57 machines (because in general OSes like to have a physical 1:1 map into the kernel part of the virtual address space, so having a higher MAXPHYADDR would be of limited use with 48-bit linear addresses). In other words, while this issue has existed forever it could be ignored until IceLake introduced MAXPHYADDR==52 machines. I'll introduce something like this in a commit message. Paolo
