On Wed, 2013-02-20 at 23:29 -0500, Kevin O'Connor wrote:
On Fri, Feb 15, 2013 at 02:11:41PM -0700, Alex Williamson wrote:
q35/ich9 doesn't use the same interrupt mapping function as i440fx/piix. PIRQA:D and PIRQE:H are programmed identically, but we start at index 0, not index -1. Slots 25 through 31 are also programmed independently.
When running qemu w/o this patch, a device at address 0:6.0 will have its PCI interrupt line register programmed with irq 10 (as seen by info pci), but it actually uses irq 11 (as reported the guest). Half of the interrupt lines are misprogrammedi like this. Functionally, a fully emulated qemu guest doesn't care much, but when we try to use device assignment, we really need to know the correct irqs.
Thanks. Before committing this, I have a couple of questions.
[...]
--- a/src/pciinit.c +++ b/src/pciinit.c @@ -91,8 +91,10 @@ const u8 pci_irqs[4] = { 10, 10, 11, 11 };
+static int (*pci_slot_get_irq)(struct pci_device *pci, int pin);
This looks like it will cause a null pointer dereference if (for what ever odd reason) neither the piix nor mch chipset is detected.
That's correct. We could initialize it to a dummy function that just does a dprintf. I can send a follow-up patch if that sounds ok.
[...]
+static int mch_pci_slot_get_irq(struct pci_device *pci, int pin) +{
- int irq, slot, pin_addend = 0;
- while (pci->parent != NULL) {
pin_addend += pci_bdf_to_dev(pci->bdf);pci = pci->parent;- }
- slot = pci_bdf_to_dev(pci->bdf);
- switch (slot) {
- /* Slots 0-24 rotate slot:pin mapping similar to piix above, but
with a different starting index - see q35-acpi-dsdt.dsl */- case 0 ... 24:
irq = pci_irqs[(pin - 1 + pin_addend + slot) & 3];break;- /* Slots 25-31 all use LNKA mapping (or LNKE, but A:D = E:H) */
- case 25 ... 31:
irq = pci_irqs[(pin - 1 + pin_addend) & 3];break;- }
I'm not sure I understand this. Since the pin to irq mapping for each pci slot is motherboard dependent (and not chipset dependent), why would the qemu "motherboard" use such a weird mapping? Why wouldn't we just have qemu do something more consistent?
PCI of course has 4 interrupt lines. 440fx only supports 4 interrupt lines, so to attempt to evenly distribute device interrupts over those 4 lines, we shift the connections on each slot. Q35 supports 8 interrupt lines. My take on this distribution is that we're reserving 4 of them for root complex devices (everything directly attached to the host bridge) and use the other 4 for add-in devices (everything behind PCIe root ports or the PCIe-to-PCI bridge). Besides load balancing, an obvious benefit for a vendor to do this kind of layout is that it reduces the chances of sharing interrupts between onboard devices and plugin devices. I'd guess that this layout likely came from looking at real hardware and trying to do something similar. Thanks,
Alex