Devices May Support Both MSI and Legacy Interrupts
When a PCI Express device supports both INTx messages and MSI, only one of the mechanism will be enabled at any given time. The most likely type of device that would support both capabilities is a boot device. A system in which a boot device resides may not support MSI during the boot sequence. Consequently configuration software will initialize interrupts by loading a Line register value and enabling the device for legacy operation just as is done for PCI devices. Once the OS loads, the device's MSI register set is loaded and the MSI enable bit is set. Setting this bit disables the devices ability to use INTx messages and enables the delivery of MSIs.
Note also that setting the Interrupt Disable ...
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