

[Xen-devel] [PATCH 00/25 v6] SBSA UART emulation support in Xen

SBSA UART emulation for guests in Xen
Linaro has published VM System specification for ARM Processors, which
provides a set of guidelines for both guest OS and hypervisor implementations, 
such that building OS images according to these guidelines guarantees
that those images can also run on hypervisors compliant with this specification.

One of the spec requirements is that the hypervisor must provide an
emulated SBSA UART as a serial console which meets the minimum requirements in 
SBSA UART as defined in appendix B of the following 
ARM Server Base Architecture Document:


This feature allows the Xen guests to use SBSA compliant pl011 UART as 
as a console. 

Note that SBSA pl011 UART is a subset of full featured ARM pl011 UART and
supports only a subset of registers as mentioned below. It does not support
rx/tx DMA.

Currently, Xen supports paravirtualized (aka PV console) and an emulated serial 
consoles. This feature will expose an emulated SBSA pl011 UART console to the
guest, which a user can access using xenconsole.

The device tree passed to the guest VM will contain the pl011 MMIO address 
range and an irq for receiving rx/tx pl011 interrupts. The device tree format 
is specified in Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt.

The Xen hypervisor will expose two types of interfaces to the backend and domU. 

The interface exposed to domU will be an emulated pl011 UART by emulating the 
access to the following pl011 registers by the guest.

- Data register (DR)            - RW
- Raw interrupt status register (RIS)   - RO
- Masked interrupt status register (MIS)- RO
- Interrupt Mask (IMSC)         - RW
- Interrupt Clear (ICR)         - WO

It will also inject the pl011 interrupts to the guest in the following 

- incoming data in the rx buffer for the guest
- there is space in the tx buffer for the guest to write more data

The interface exposed to the backend will be the same PV console interface, 
which minimizes the changes required in xenconsole to support a new pl011 

This interface has rx and tx ring buffers and an event channel for 
sending/receiving events from the backend. 

So essentially Xen handles the data on behalf of domU and the backend. Any data 
written by domU is captured by Xen and written to the TX (OUT) ring buffer 
and a pl011 event is raised to the backend to read the TX ring buffer.
Similarly on reciving a pl011 event, Xen injects an interrupt to guest to
indicate there is data available in the RX (IN) ring buffer.

The pl011 UART state is completely captured in the set of registers 
mentioned above and this state is updated everytime there is an event from 
the backend or there is register read/write access from domU. 

For example, if domU has masked the rx interrupt in the IMSC register, then Xen 
will not inject an interrupt to guest and will just update the RIS register. 
Once the interrupt is unmasked by guest, the interrupt will be delivered to the 

Changes summary:

Xen Hypervisor

1. Add emulation code to emulate read/write access to pl011 registers and pl011 
    - It emulates DR read/write by reading and writing from/to the IN and 
      OUT ring buffers and raising an event to dom0 when there is data in 
      the OUT ring buffer and injecting an interrupt to the guest when there 
      is data in the IN ring buffer.
    - Other registers are related to interrupt management and essentially 
      control when interrupts are delivered to the guest.

2. Add a new domctl API to initialize vpl011 emulation in Xen.

3. Enable vpl011 emulation for a domain based on a libxl option passed during 
   domain creation.


1. Add a new option "vuart" in the domU configuration file to enable/disable 

2. Create a SBSA UART DT node in the guest device tree. It uses a fixed
   vpl011 SPI IRQ number and MMIO address.

3. Call vpl011 init DOMCTL API to enable vpl011 emulation.

5. Add a new vuart xenstore node, which contains:
    - ring-ref
    - event channel
    - buffer limit
    - type


1. Split the domain structure to support multiple consoles.

2. Modify different APIs such as buffer_append() etc. to operate on the 
   console structure.
3. Add support for handling multiple consoles.

4. Add support for vuart console:

The vpl011 changes available at the following repo:

url: https://git@xxxxxxxxxxxxxx:/people/bhupinder.thakur/xen.git
branch: vpl011_v6

Kindly wait for one day to checkout the code from the above URL.

There are some TBD items which need to be looked at in the future:

1. Currently UEFI firmware logs the output to hvc console only. How can 
   UEFI firmware be made aware of pl011 console and how it can use it
   as a console instead of hvc.

   There was a discussion on this and it was decided that SBSA UART should 
   be used as a debug port by the UEFI firmware so that all debug output
   is redirected to this port.

2. Linux seems to have hvc console as the default console i.e. if no
   console is specified then it uses hvc as the console. How can an 
   option be provided in Linux to select either hvc or pl011 as the 
   default console.

   It was suggeted to use the SPCR in ACPI and the stdout-path option in the
   device tree to specify the default console. However, currently hvc console
   is not describable in the ACPI/device tree. This support will have to be
   added to allow the user to specify the default console.

3. ACPI support for pl011 device.

CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

Bhupinder Thakur (25):
  xen/arm: vpl011: Define common ring buffer helper functions in
  xen/arm: vpl011: Add SBSA UART emulation in Xen
  xen/arm: vpl011: Allocate a new GFN in the toolstack for vuart
  xen/arm: vpl011: Add support for vuart in libxl
  xen/arm: vpl011: Rearrange xen header includes in alphabetical order
    in domctl.c
  xen/arm: vpl011: Add a new domctl API to initialize vpl011
  xen/arm: vpl011: Add a new vuart node in the xenstore
  xen/arm: vpl011: Modify xenconsole to define and use a new console    
  xen/arm: vpl011: Rename the console structure field conspath to xspath
  xen/arm: vpl011: Modify xenconsole functions to take console structure
    as input
  xen/arm: vpl011: Add a new console_init function in xenconsole
  xen/arm: vpl011: Add a new buffer_available function in xenconsole
  xen/arm: vpl011: Add a new add_console_evtchn_fd function in
  xen/arm: vpl011: Add a new add_console_tty_fd function in xenconsole
  xen/arm: vpl011: Add a new console_evtchn_unmask function in
  xen/arm: vpl011: Add a new handle_console_ring function in xenconsole
  xen/arm: vpl011: Add a new handle_console_tty function in xenconsole
  xen/arm: vpl011: Add a new console_cleanup function in xenconsole
  xen/arm: vpl011: Add a new console_open_log function in xenconsole
  xen/arm: vpl011: Add a new console_close_evtchn function in xenconsole
  xen/arm: vpl011: Add support for multiple consoles in xenconsole
  xen/arm: vpl011: Add support for vuart console in xenconsole
  xen/arm: vpl011: Add a new vuart console type to xenconsole client
  xen/arm: vpl011: Add a pl011 uart DT node in the guest device tree
  xen/arm: vpl011: Update documentation for vuart console support

 config/arm32.mk                      |   1 +
 config/arm64.mk                      |   1 +
 docs/man/xl.cfg.pod.5.in             |   9 +
 docs/misc/console.txt                |  44 ++-
 tools/console/Makefile               |   3 +-
 tools/console/client/main.c          |  13 +-
 tools/console/daemon/io.c            | 662 ++++++++++++++++++++++++-----------
 tools/libxc/include/xc_dom.h         |   2 +
 tools/libxc/include/xenctrl.h        |  18 +
 tools/libxc/xc_dom_arm.c             |   5 +-
 tools/libxc/xc_dom_boot.c            |   2 +
 tools/libxc/xc_domain.c              |  24 ++
 tools/libxl/libxl.h                  |   6 +
 tools/libxl/libxl_arch.h             |   6 +
 tools/libxl/libxl_arm.c              |  72 +++-
 tools/libxl/libxl_console.c          |  47 +++
 tools/libxl/libxl_create.c           |   9 +-
 tools/libxl/libxl_device.c           |   9 +-
 tools/libxl/libxl_dom.c              |   5 +
 tools/libxl/libxl_internal.h         |   6 +
 tools/libxl/libxl_types.idl          |   7 +
 tools/libxl/libxl_types_internal.idl |   1 +
 tools/libxl/libxl_x86.c              |   8 +
 tools/xl/xl_cmdtable.c               |   2 +-
 tools/xl/xl_console.c                |   5 +-
 tools/xl/xl_parse.c                  |   8 +
 xen/arch/arm/Kconfig                 |   7 +
 xen/arch/arm/Makefile                |   1 +
 xen/arch/arm/domain.c                |   5 +
 xen/arch/arm/domctl.c                |  43 ++-
 xen/arch/arm/vpl011.c                | 455 ++++++++++++++++++++++++
 xen/include/asm-arm/domain.h         |   6 +
 xen/include/asm-arm/pl011-uart.h     |   2 +
 xen/include/asm-arm/vpl011.h         |  72 ++++
 xen/include/public/arch-arm.h        |   6 +
 xen/include/public/domctl.h          |  21 ++
 xen/include/public/io/console.h      |   4 +
 37 files changed, 1360 insertions(+), 237 deletions(-)
 create mode 100644 xen/arch/arm/vpl011.c
 create mode 100644 xen/include/asm-arm/vpl011.h


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