# QEMU<->ACPI BIOS NVDIMM interface

QEMU supports NVDIMM via ACPI. This document describes the basic concepts of NVDIMM ACPI and the interface between QEMU and the ACPI BIOS.

## NVDIMM ACPI Background

NVDIMM is introduced in ACPI 6.0 which defines an NVDIMM root device under _SB scope with a _HID of "ACPI0012". For each NVDIMM present or intended to be supported by platform, platform firmware also exposes an ACPI Namespace Device under the root device.

The NVDIMM child devices under the NVDIMM root device are defined with _ADR corresponding to the NFIT device handle. The NVDIMM root device and the NVDIMM devices can have device specific methods (_DSM) to provide additional functions specific to a particular NVDIMM implementation.

This is an example from ACPI 6.0, a platform contains one NVDIMM:

    Scope (\_SB){
       Device (NVDR) // Root device
       {
          Name (_HID, "ACPI0012")
          Method (_STA) {...}
          Method (_FIT) {...}
          Method (_DSM, ...) {...}
          Device (NVD)
          {
             Name(_ADR, h) //where h is NFIT Device Handle for this NVDIMM
             Method (_DSM, ...) {...}
          }
       }
    }

### Methods supported on both NVDIMM root device and NVDIMM device

_DSM (Device Specific Method)

  It is a control method that enables devices to provide device specific control functions that are consumed by the device driver. The NVDIMM DSM specification can be found at http://pmem.io/documents/NVDIMM_DSM_Interface_Example.pdf

  Arguments:

  Arg0

    A Buffer containing a UUID (16 Bytes)

  Arg1

    An Integer containing the Revision ID (4 Bytes)

  Arg2

    An Integer containing the Function Index (4 Bytes)

  Arg3

    A package containing parameters for the function specified by the UUID, Revision ID, and Function Index

  Return Value:

  If Function Index = 0, a Buffer containing a function index bitfield. Otherwise, the return value and type depends on the UUID, revision ID and function index which are described in the DSM specification.

### Methods on NVDIMM ROOT Device

_FIT(Firmware Interface Table)

  It evaluates to a buffer returning data in the format of a series of NFIT Type Structure.

  Arguments: None

  Return Value: A Buffer containing a list of NFIT Type structure entries.

  The detailed definition of the structure can be found at ACPI 6.0: 5.2.25 NVDIMM Firmware Interface Table (NFIT).

## QEMU NVDIMM Implementation

QEMU uses 4 bytes IO Port starting from 0x0a18 and a RAM-based memory page for NVDIMM ACPI.

Memory:

  QEMU uses BIOS Linker/loader feature to ask BIOS to allocate a memory page and dynamically patch its address into an int32 object named "MEMA" in ACPI.

  This page is RAM-based and it is used to transfer data between _DSM method and QEMU. If ACPI has control, this pages is owned by ACPI which writes _DSM input data to it, otherwise, it is owned by QEMU which emulates _DSM access and writes the output data to it.

  ACPI writes _DSM Input Data (based on the offset in the page):

  [0x0 - 0x3]

    4 bytes, NVDIMM Device Handle.

    The handle is completely QEMU internal thing, the values in range [1, 0xFFFF] indicate nvdimm device. Other values are reserved for other purposes.

    Reserved handles:

    - 0 is reserved for nvdimm root device named NVDR.

    - 0x10000 is reserved for QEMU internal DSM function called on the root device.

  [0x4 - 0x7]

    4 bytes, Revision ID, that is the Arg1 of _DSM method.

  [0x8 - 0xB]

    4 bytes. Function Index, that is the Arg2 of _DSM method.

  [0xC - 0xFFF]

    4084 bytes, the Arg3 of _DSM method.

  QEMU writes Output Data (based on the offset in the page):

  [0x0 - 0x3]

    4 bytes, the length of result

  [0x4 - 0xFFF]

    4092 bytes, the DSM result filled by QEMU

IO Port 0x0a18 - 0xa1b:

  ACPI writes the address of the memory page allocated by BIOS to this port then QEMU gets the control and fills the result in the memory page.

  Write Access:

  [0x0a18 - 0xa1b]

    4 bytes, the address of the memory page allocated by BIOS.

## _DSM process diagram

"MEMA" indicates the address of memory page allocated by BIOS.

    +----------------------+      +-----------------------+
    |    1. OSPM           |      |    2. OSPM            |
    | save _DSM input data |      |  write "MEMA" to      | Exit to QEMU
    | to the page          +----->|  IO port 0x0a18       +------------+
    | indicated by "MEMA"  |      |                       |            |
    +----------------------+      +-----------------------+            |
                                                                       |
                                                                       v
    +--------------------+       +-----------+      +------------------+--------+
    |      5 QEMU        |       | 4 QEMU    |      |        3. QEMU            |
    | write _DSM result  |       |  emulate  |      | get _DSM input data from  |
    | to the page        +<------+ _DSM      +<-----+ the page indicated by the |
    |                    |       |           |      | value from the IO port    |
    +--------+-----------+       +-----------+      +---------------------------+
             |
             | Enter Guest
             |
             v
    +--------------------------+      +--------------+
    |     6 OSPM               |      |   7 OSPM     |
    | result size is returned  |      |  _DSM return |
    | by reading  DSM          +----->+              |
    | result from the page     |      |              |
    +--------------------------+      +--------------+

## NVDIMM hotplug

ACPI BIOS GPE.4 handler is dedicated for notifying OS about nvdimm device hot-add event.

## QEMU internal use only _DSM functions

### Read FIT

_FIT method uses _DSM method to fetch NFIT structures blob from QEMU in 1 page sized increments which are then concatenated and returned as _FIT method result.

Input parameters:

Arg0

  UUID {set to 648B9CF2-CDA1-4312-8AD9-49C4AF32BD62}

Arg1

  Revision ID (set to 1)

Arg2

  Function Index, 0x1

Arg3

  A package containing a buffer whose layout is as follows:

  > Field | Length | Offset | Description
  > offset | 4 | 0 | offset in QEMU's NFIT structures blob to read from

Output layout in the dsm memory page:

> Field | Length | Offset | Description
> length | 4 | 0 | length of entire returned data (including this header)

> status

> 4

> 4

> return status codes

> - 0x0 - success

> - 0x100 - error caused by NFIT update while read by _FIT wasn't completed

> - other codes follow Chapter 3 in DSM Spec Rev1

> fit data | Varies | 8 | contains FIT data. This field is present if status field is 0.

The FIT offset is maintained by the OSPM itself, current offset plus the size of the fit data returned by the function is the next offset OSPM should read. When all FIT data has been read out, zero fit data size is returned.

If it returns status code 0x100, OSPM should restart to read FIT (read from offset 0 again).