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

Xiaoyao Li
2024-04-07T03:15:08Z

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?

> [1]https://drive.google.com/corp/drive/folders/1hm_ITeuB6DjT7dNd-6Ezybio4tRRQOlC
> [2]https://lore.kernel.org/all/20240313125844.912415-1-kraxel@redhat.com