Thursday 23 January 2014

Understanding PCI Configuration Space

I noticed in a dump file I was debugging for a user on Sysnative Forums, within the call stack there was a few references to PCI Configuration Space. The PCI Configuration Space can be accessed by device drivers and other programs which use software drivers to gather additional information. The call stack in the example was easy to find a possible cause, however, the topic of this discussion will be explaining the PCI Configuration Space.

The driver in question belongs to CPU-Z.

PCI Configuration Space

The PCI Configuration Space is a set of registers, on PCI Express (PCIe) buses, this configuration space may be referred to as the the Extended Configuration Space. These registers are then mapped to memory locations such as the I/O Address Space of the CPU.

 The Configuration Space is typically 256 bytes, and can be accessed with Read/Write Configuration Cycles. The target device for the Configuration Space Access is selected with the Initialization Device Select (IDSEL) signal, which is then decoded by the target device. Although, in order to make the PCI Configuration Space accessible (most CPUs do not support such access), there must be kind of mechanism, which can allow access to the PCI Configuration Space. This mechanism is divided into two parts: Mechanism #1 and Mechanism #2. Mechanism #2 is only provided for backwards compatibility, and therefore will not be part of the discussion later on.

The Configuration Space can be accessed by drivers with a set IRPs, or by accessing members within the BUS_INTERFACE_STANDARD data structure.

 The IRP_MN_WRITE_CONFIG is used to to write data to the configuration space specified. The IRP is a Minor Function Code for a PnP IRP. Since the buffer containing the information to be written is allocated from paged pool, the bus driver must call this IRP at IRQL lower than DISPATCH_LEVEL.

 The IRP_MN_READ_CONFIG is used to read data from the configuration space. This is IRP is also the Minor Function Code for a PnP IRP, and also must be called at IRQL level lower than DISPATCH_LEVEL for the same reasons above.

I mentioned earlier about the two types of Mechanisms used to access the Configuration Space. With the newer mechanism, two 32-bit I/O registers are created, one is called CONFIG_ADDRESS (0xCF8) and is used to provide the configuration address to be accessed, whereas, the second is named CONFIG_DATA and is used to transfer data to and from the CONFIG_DATA register.

The structure of the CONFIG_ADDRESS register can be found below:

Bit 31 is used to enable the translation of accesses to CONFIG_DATA to configuration cycles. These cycles are simply addresses on the PCI/PCIe bus. These Configuration Cycles are divided into two types: Type 1 and Type 2, and will be discussed later on.

The Bits 23-16 are used to serve as a Bus Number, to be able to enable the system to select a specific PCI bus. The Bits 15-11 are used to serve as a Device Number, to select a device connected to the PCI bus. Bits 10-8 are used for selection of PCI functions (if more than one), which are in simple terms logical devices for a certain device. The remaining bits mainly depend upon alignment requirements, but the last two bits must be 0, since all reads and writes have to be 32-bits long.

We haven't discussed one of the more fundamental aspects of the Configuration Space, which is the Configuration Space Header. The diagram below shows the structure of the said header.

The Configuration Space Header provides information to enable the operating system to interact and control the device. There is currently two different Header Types, which are Type 1 for bridges, switches and root complexes. Type 0 for Endpoints. The above example is for Endpoints.

The Interrupt Pin and Interrupt Lines are used with IRQs, IDT and IRQL Levels. Each PCI device typically has four pins which are A, B, C and D. Each pin is used to enable the hardware to interrupt the processor, using the Interrupt Line, which is used to map to a ISR within the IDT, so the appropriate ISR routine can be handled the device driver code.

The Device ID and Vendor ID are used to identify the device connected to the PCI bus. We can use these values in a PCI Database to find the manufacturer of the device.

The Subsystem Vendor ID (SVID) and Subsystem ID (SSID) provide model specific device information.

 The Command and Status registers are used to provide feature and error reporting information which I have discussed in my Stop 0x124 PCIe Debugging Posts.

The BIST register is set by changing Bit 6 to 1, and then performing a Built-In Self Test.

We can view the Configuration Space for a certain device with WinDbg, which I will show later in the WinDbg Extensions section of this post. I'll explain Base Address Registers, Class Codes and Bus Enumeration there too.
Call Stack Functions

I've managed to find some code on ReactOS, which may explain some of the internal PCI functions seen in the call stack of the dump file.

HalpPCIConfig seems to perform some form of synchronization, and then begins performing a read/write operation with the specified buffer.

The rest of the documentation can be found here.

WinDbg Extensions

I've managed to find two extensions which could provide additional information with our debugging efforts. These extensions are !pcitree and !pci. Please note in order to use !pci, you will need a Live Debugging sessions with either the local computer or do it remotely. I enabled a Live Debugging session on my computer to provide you with an example.

The !pcitree extensions shows all the buses and the devices attached to these buses. Using the information provided with the !devext extension, the other fields provided by the !pcitree extension will become more apparent.

 The d field is the Device Number, the f field is the Function Number. We can see the Interrupt Line number, the Vendor and Device information, Header Type and Class Information.

The !pci extension with the 0x100 flag shows the information stored within the PCI Configuration Space, please note this is a omitted output of one particular PCI bus.

There's three key points I would like to discuss in this section. BARs, Classes and Bus Enumeration. I will start with Bus Enumeration which we can obtain with the !pcitree extension.

 There are three different methods of Bus Enumeration, which will described shortly. Bus Enumeration is a method of checking any devices are connected to the PCI bus. This is required since the BIOS and the operating system do not have any method of their own to be able to check if a device is connected. Bus Enumeration is performed by checking the Vendor ID and Device ID Registers, and then returning F's (Hexadecimal) and 1's (Binary) if the reading of the Function and Register fail.

Now, let's briefly look at the concept of BARs or (Base Address Registers). BARs are used to define the memory addresses and I/O port addresses (memory-mapped I/O) used by the PCI devices. These registers define the size of memory to be allocated, and where it can be allocated. Typically, Memory BARs must be in physical memory, whereas, I/O BARs do not have this restriction.

0x02 in the Type field means the address is 32-bit, 0x00 means the address is 64-bit and 0x01 is Reserved.

The last concept I'm going to discuss involves Classes. Classes are separated into Class Code, Sub Class Code and Prog IF (Programmable Interface) and are used to identify the type of device and it's potential functions.

There a larger number of Sub Classes and Functions, so I'll have to omit the complete listing, and refer you to the PCI OSDev Wiki article.

Additional Reading

PCI - OSDev Wiki
PCI Configuration Space
PCI Internals
OSR NtDev List - PCI Function Number

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