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

Paolo Bonzini
2024-01-11T20:02:34Z

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.

Paolo