Re: [PATCH 0/5] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

[Jim Mattson]

On Thu, Feb 27, 2020 at 9:23 AM Mohammed Gamal <mgamal@xxxxxxxxxx> wrote:
>
> When EPT/NPT 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.
>
> 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.

What about the #GP that should be delivered if any reserved bits are
set in any of the 4 PDPTRs when the guest loads CR3 in PAE mode?