Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

Jim Mattson
2024-01-11T21:12:41Z

On Thu, Jan 11, 2024 at 12:02 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On Thu, Jan 11, 2024 at 5:25 PM Sean Christopherson <seanjc@google.com> wrote:
> > > It is unusual to assign a huge RAM to guest, but passthrough a device also may trigger
> > > this issue which we have met, i.e. alloc memslot for the 64bit BAR which can set
> > > bits[51:48]. BIOS can control the BAR address, e.g. seabios moved 64bit pci window
> > > to end of address space by using advertised physical bits[1].
> >
> > Drat. Do you know if these CPUs are going to be productized? We'll still need
> > something in KVM either way, but whether or not the problems are more or less
> > limited to funky software setups might influence how we address this.
>
> Wait, we do have an API for guest physical address size. It's
> KVM_GET_SUPPORTED_CPUID2: the # of bits is in leaf 0x80000008, bits
> 0:7 of EAX. In fact that leaf is what firmware uses to place the BARs.
> So it just needs to be adjusted for VMX in __do_cpuid_func, and looked
> up in selftests.

We've discussed this. The only *supported* value for guest.MAXPHYADDR
is host.MAXPHYADDR.

If EPT doesn't also support that value, then the only *supported*
configuration is shadow paging.

If someone wants to run with scissors, that's fine, but don't abuse
KVM_GET_SUPPORTED_CPUID2 to return an *unsupported* configuration.