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+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 to access it. pseries guests on such
+systems can communicate with the ultravisor (via ultracalls) to switch to a
+secure VM mode (SVM) 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
+public OpenPOWER version of the PAPR specification (LoPAPR/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 internally-maintained PAPR
+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 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)
+
+  For TPM_COMM_OP_EXECUTE operation:
+    Send a request to a TPM and receive a response, opening a new TPM session
+    if one has not already been opened.
+
+  For TPM_COMM_OP_CLOSE_SESSION operation:
+    Close the existing TPM session, if any.
+
+  Arguments:
+
+    r3 : H_TPM_COMM (0xef10)
+    r4 : TPM operation, one of:
+         TPM_COMM_OP_EXECUTE (0x1)
+         TPM_COMM_OP_CLOSE_SESSION (0x2)
+    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 4KB
+    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 4KB, 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 : 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.
+
+  Use-case/notes:
+
+    SVM filesystems 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.