Question about Message Signaled Interrupts (MSI) on x86 LAPIC system

Question

Hi I'm writing a kernel and plan to use MSI interrupt for PCI devices.

However, I'm also quite confused by the documentations.

My understanding about MSI are as follow:

From PCI device point of view:

1. Documentations indicate that I need to find Capabillty ID = 0x05 to locate 3 registers: Message control (MCR), Message Address (MAR) and Message Data (MDR) registers
2. MCR provide control functionality for MSI interrupt,
3. MAR provide the physical address the PCI device will write once interrupt occurs
4. MDR forms out the actual data it will write into the physical address

From CPU point of view:

1. Documentation shows that Message Address register contains fixed top of 0xFEE, and following by destination ID (LAPIC ID) and other controlling bits as follow:

2. The Message Data register will contain the following information, including the interrupt vector:

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After reading all of these, I am thinking if the APIC_ID is 0x0h would the Message Address conflict with the Local APIC memory mapping? Although the address of FEE00000~FEE00010 are reserved.

In addition, is it true that the vector number in MDR is corresponding to the IDT vector number. In other words, if I put MAR = 0xFEE0000C (Destination ID = 0, Using logical APIC ID) and MDR = 0x0032 (edge trigger, Vector = 50) and enable the MSI interrupt, then once the device issues an interrupt CPU would correspondingly run the function pointed by IDT[50]? After that I write 0h to EOI register to end it?

Finally, according to the documentation, the upper 32 bit of MAR is not used? Can anyone help on this?

Thanks a lot!

Quar

@prl the Message Control Register provide the ability to use 32 bit or 64 bit addressing, does it means I do not need to set it to 64 bit addressing model?? – Quar

@prl Furthermore, what if I re-map the LAPIC address and manually make the addressing conflict?? What would happen? – Quar

@prl So if I set MAR = 0xFEE0000C and MDR = 0x0032, the device will invoke the IDT[50] right?? And would you like to post your answer and I'll accept it. – Quar

Answer 1

Your understanding of how to detect and program MSI in a PCI (or PCIe) device is correct.* The message address controls the destination (which CPU the interrupt is sent to), while the message data contains the vector number. For normal interrupts, all bits of the message data should be 0 except for the low 8 bits, which contain the vector. The vector is an index into the IDT, so if the message data is 0x0032, the interrupt is delivered through entry 50 of the IDT.**

If the Destination ID in an interrupt message is 0, the Message Address of the MSI does match the default address of the local APIC, but they do not conflict, because the APIC can only be written by the CPU and MSIs can only be written by devices.

On x86 platforms, the upper 32 bits of the message address must be 0. This can be done by setting the upper part of the message address to 0 or by programming the device to use a 32-bit message address (in which case the upper message address register is not used). The PCI spec was designed to work with systems where 64-bit MSI addresses are used, but x86 systems never use the upper 32 bits of the message address.

Reprogramming the APIC base address by writing to the APIC_BASE MSR does not affect the address range used for MSI; it is always 0xFEExxxxx.

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* You should also look at the MSI-X capability, because some devices support MSI-X but not MSI. MSI-X is a bit more flexible, which inevitably makes it a bit more complicated.

** When using the MSI capability, the message data isn't exactly the value in the Message Data Register (MDR). The MSI capability allows the device to use several contiguous vectors. When the device sends an interrupt message, it replaces the low bits of the MDR with a different value depending on the interrupt cause within the device.

Pinkard