Hypervisor calls and the Ultravisor

On PPC64 systems supporting Protected Execution Facility (PEF), system memory can be placed in a secured region where only an ultravisor running in firmware can provide access to. pSeries guests on such systems can communicate with the ultravisor (via ultracalls) to switch to a secure virtual machine (SVM) mode where the guest’s memory is relocated to this secured region, making its memory inaccessible to normal processes/guests running on the host.

The various ultracalls/hypercalls relating to SVM mode are currently only documented internally, but are planned for direct inclusion into the Linux on Power Architecture Reference document ([LoPAR]). An internal ACR has been filed to reserve a hypercall number range specific to this use case to avoid any future conflicts with the IBM internally maintained Power Architecture Platform Reference (PAPR+) documentation specification. This document summarizes some of these details as they relate to QEMU.

Hypercalls needed by the ultravisor

Switching to SVM mode involves a number of hcalls issued by the ultravisor to the hypervisor to orchestrate the movement of guest memory to secure memory and various other aspects of the SVM mode. Numbers are assigned for these hcalls within the reserved range 0xEF00-0xEF80. The below documents the hcalls relevant to QEMU.

H_TPM_COMM (0xef10)

SVM file systems are encrypted using a symmetric key. This key is then wrapped/encrypted using the public key of a trusted system which has the private key stored in the system’s TPM. An Ultravisor will use this hcall to unwrap/unseal the symmetric key using the system’s TPM device or a TPM Resource Manager associated with the device.

The Ultravisor sets up a separate session key with the TPM in advance during host system boot. All sensitive in and out values will be encrypted using the session key. Though the hypervisor will see the in and out buffers in raw form, any sensitive contents will generally be encrypted using this session key.


r3: H_TPM_COMM (0xef10)

r4: TPM operation, one of:

TPM_COMM_OP_EXECUTE (0x1): send a request to a TPM and receive a response, opening a new TPM session if one has not already been opened.

TPM_COMM_OP_CLOSE_SESSION (0x2): close the existing TPM session, if any.

r5: in_buffer, guest physical address of buffer containing the request. Caller may use the same address for both request and response.

r6: in_size, size of the in buffer. Must be less than or equal to 4 KB.

r7: out_buffer, guest physical address of buffer to store the response. Caller may use the same address for both request and response.

r8: out_size, size of the out buffer. Must be at least 4 KB, as this is the maximum request/response size supported by most TPM implementations, including the TPM Resource Manager in the linux kernel.

Return values:

r3: one of the following values:

H_Success: request processed successfully.

H_PARAMETER: invalid TPM operation.

H_P2: in_buffer is invalid.

H_P3: in_size is invalid.

H_P4: out_buffer is invalid.

H_P5: out_size is invalid.

H_RESOURCE: problem communicating with TPM.

H_FUNCTION: TPM access is not currently allowed/configured.

r4: For TPM_COMM_OP_EXECUTE, the size of the response will be stored here upon success.