Re: [ANNOUNCE] PUCK Notes - 2024.04.03 - TDX Upstreaming Strategy

Sean Christopherson
2024-04-09T14:01:08Z

On Tue, Apr 09, 2024, Xiaoyao Li wrote:
> On 4/9/2024 12:20 AM, Sean Christopherson wrote:
> > On Sun, Apr 07, 2024, Xiaoyao Li wrote:
> > > On 4/6/2024 12:58 AM, Sean Christopherson wrote:
> > > > - For guest MAXPHYADDR vs. GPAW, rely on KVM_GET_SUPPORTED_CPUID to enumerate
> > > > the usable MAXPHYADDR[2], and simply refuse to enable TDX if the TDX Module
> > > > isn't compatible. Specifically, if MAXPHYADDR=52, 5-level paging is enabled,
> > > > but the TDX-Module only allows GPAW=0, i.e. only supports 4-level paging.
> > >
> > > So userspace can get supported GPAW from usable MAXPHYADDR, i.e.,
> > > CPUID(0X8000_0008).eaxx[23:16] of KVM_GET_SUPPORTED_CPUID:
> > > - if usable MAXPHYADDR == 52, supported GPAW is 0 and 1.
> > > - if usable MAXPHYADDR <= 48, supported GPAW is only 0.
> > >
> > > There is another thing needs to be discussed. How does userspace configure
> > > GPAW for TD guest?
> > >
> > > Currently, KVM uses CPUID(0x8000_0008).EAX[7:0] in struct
> > > kvm_tdx_init_vm::cpuid.entries[] of IOCTL(KVM_TDX_INIT_VM) to deduce the
> > > GPAW:
> > >
> > > int maxpa = 36;
> > > entry = kvm_find_cpuid_entry2(cpuid->entries, cpuid->nent, 0x80000008, 0);
> > > if (entry)
> > > max_pa = entry->eax & 0xff;
> > >
> > > ...
> > > if (!cpu_has_vmx_ept_5levels() && max_pa > 48)
> > > return -EINVAL;
> > > if (cpu_has_vmx_ept_5levels() && max_pa > 48) {
> > > td_params->eptp_controls |= VMX_EPTP_PWL_5;
> > > td_params->exec_controls |= TDX_EXEC_CONTROL_MAX_GPAW;
> > > } else {
> > > td_params->eptp_controls |= VMX_EPTP_PWL_4;
> > > }
> > >
> > > The code implies that KVM allows the provided CPUID(0x8000_0008).EAX[7:0] to
> > > be any value (when 5level ept is supported). when it > 48, configure GPAW of
> > > TD to 1, otherwise to 0.
> > >
> > > However, the virtual value of CPUID(0x8000_0008).EAX[7:0] inside TD is
> > > always the native value of hardware (for current TDX).
> > >
> > > So if we want to keep this behavior, we need to document it somewhere that
> > > CPUID(0x8000_0008).EAX[7:0] in struct kvm_tdx_init_vm::cpuid.entries[] of
> > > IOCTL(KVM_TDX_INIT_VM) is only for configuring GPAW, not for userspace to
> > > configure virtual CPUID value for TD VMs.
> > >
> > > Another option is that, KVM doesn't allow userspace to configure
> > > CPUID(0x8000_0008).EAX[7:0]. Instead, it provides a gpaw field in struct
> > > kvm_tdx_init_vm for userspace to configure directly.
> > >
> > > What do you prefer?
> >
> > Hmm, neither. I think the best approach is to build on Gerd's series to have KVM
> > select 4-level vs. 5-level based on the enumerated guest.MAXPHYADDR, not on
> > host.MAXPHYADDR.
>
> I see no difference between using guest.MAXPHYADDR (EAX[23:16]) and using
> host.MAXPHYADDR (EAX[7:0]) to determine the GPAW (and EPT level) for TD
> guest. The case for TDX diverges from what for non TDX VMs. The value of
> them passed from userspace can only be used to configure GPAW and EPT level
> for TD, but won't be reflected in CPUID inside TD.

But the TDX module will emulate EAX[7:0] to match hardware, no? Whenever possible,
the CPUID entries passed to KVM should match the CPUID values that are observed
by the guest. E.g. if host.MAXPHYADDR=52, but the CPU only supports 4-level
paging, then KVM should get host.MAXPHYADDR=52, guest.MAXPHYADDR=48.

As I said in my response to Rick:

: > An alternative would be to have the KVM API peak at the value, and then
: > discard it (not pass the leaf value to the TDX module). Not ideal.
:
: Heh, I typed up this idea before reading ahead. This has my vote. Unless I'm
: misreading where things are headed, using guest.MAXPHYADDR to communicate what
: is essentially GPAW to the guest is about to become the de facto standard.
:
: At that point, KVM can basically treat the current TDX module behavior as an
: erratum, i.e. discarding guest.MAXPHYADDR becomes a workaround for a "CPU" bug,
: not some goofy KVM quirk.