doc: add ahci virtualization introduction

Signed-off-by: Yuan Liu <yuan1.liu@intel.com>

doc/developer-guides/hld/ahci-hld.rst

@@ -0,0 +1,44 @@
+.. _ahci-hld:
+
+AHCI Virtualization in Device Model
+###################################
+
+AHCI (Advanced Host Controller Interface), which is a hardware mechanism
+that allows software to communicate with Serial ATA devices. AHCI HBA
+(host bus adapters) is a PCI class device that acts as a data movement
+engine between system memory and Serial ATA devices. The ACPI HBA in
+ACRN support both ATA and ATAPI devices. The architecture is shown in
+below diagram.
+
+.. figure:: images/ahci-image1.png
+   :align: center
+   :width: 750px
+   :name: achi-device
+
+HBA is registered to PCI system with device id 0x2821 and vendor id
+0x8086. And its memory registers are mapped in BAR 5. It only supports 6
+ports refer to ICH8 AHCI. AHCI driver in Guest OS can access HBA in DM
+through the PCI BAR. And HBA can inject MSI interrupts through PCI
+framework.
+
+When application in Guest OS reads data from /dev/sda, the request will
+send through the AHCI driver then the PCI driver. Guest VM will trap to
+hypervisor, and hypervisor dispatch the request to DM. According to the
+offset in the BAR, the request will dispatch to port control handler.
+Then the request is parse to a block I/O request which can be processed
+by Block backend model.
+
+Usage:
+
+***-s <slot>,ahci,<type:><filepath>***
+
+Type:  ‘hd’ and ‘cd’ are available.
+
+Filepath:  the path for the backend file, could be a partition or a
+regular file.
+
+E.g．
+
+    SOS: -s 20,ahci,\ `hd:/dev/mmcblk0p1 <http://hd/dev/mmcblk0p1>`__
+
+    UOS: /dev/sda

doc/developer-guides/hld/hld-emulated-devices.rst

@@ -14,4 +14,5 @@ documented in this section.
    usb-virt-hld
    UART virtualization <uart-virt-hld>
    Watchdoc virtualization <watchdog-hld>
+   AHCI virtualization <ahci-hld>
    GVT-g GPU Virtualization <hld-APL_GVT-g>