Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

Jim Mattson
2020-07-10T16:31:12Z

On Fri, Jul 10, 2020 at 8:48 AM Mohammed Gamal <mgamal@redhat.com> wrote:
>
> When EPT is enabled, KVM does not really look at guest physical
> address size. Address bits above maximum physical memory size are reserved.
> Because KVM does not look at these guest physical addresses, it currently
> effectively supports guest physical address sizes equal to the host.
>
> 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?

> In this patch series we add checks on guest physical addresses in EPT
> violation/misconfig and NPF vmexits and if needed inject the proper
> page faults in the guest.
>
> A more subtle issue is when the host MAXPHYADDR is larger than that of the
> guest. Page faults caused by reserved bits on the guest won't cause an EPT
> violation/NPF and hence we also check guest MAXPHYADDR and add PFERR_RSVD_MASK
> error code to the page fault if needed.