I just got a bugreport from a virtualbox developer who has noticed a
problem with the xHCI driver in FreeBSD 10.0. The irc transcript
follows to not loose any information from the analysis and how this
could be addressed.
(01:22:20 PM) MichalN: decke: I ran into a funny problem with the xHCI
driver in FreeBSD 10.0. It only works if the host uses MSIs, but
doesn't make sure. No such problem in 9.2.
(01:23:28 PM) MichalN: It probably doesn't come up on physical
hardware, or at least it'd require something fairly unusual (an old
system with add-on xHCI adapter).
(01:23:39 PM) MichalN: But it's a problem in a virtualized environment :)
(01:24:13 PM) MichalN: The default VirtualBox config doesn't do MSIs.
(01:25:10 PM) klaus-vb: it emulates an old system ;)
(01:25:26 PM) MichalN: The driver should either work (just like the
9.2 one does), or at least fail to load if it can't enable MSIs. As it
is, it gets an interrupt storm.
(01:25:38 PM) MichalN: (which FreeBSD duly detects and reports)
(01:26:39 PM) MichalN: Actually if I can find it, I can point you at
the code that was removed...
(01:27:07 PM) klaus-vb: the standard vbox chipset can't do MSI. only
the experimental ICH9 chipset code does MSI, and we generally don't
test it much
(01:27:52 PM) MichalN: Because there's not a huge advantage to MSIs in
a virtualized environment, unlike physical systems.
(01:29:47 PM) MichalN: OK, here's some older version of the code:
In function xhci_interrupt() the code writing the XHCI_IMAN_INTR_PEND
bit was removed in 10.0.
(01:30:51 PM) MichalN: For your background - the xHCI controller
behaves slightly differently depending on whether MSIs are used or
not. If not, that bit (called XHCI_IMAN_INTR_PEND in the FreeBSD
driver) must be written to clear an interrupt.
(01:31:22 PM) MichalN: If MSIs are used then it doesn't need to be
done. It seems someone removed the code which was unconditional and
unnecessary on systems with MSI.
(01:31:31 PM) MichalN: But that breaks on systems with no MSI.
(01:33:07 PM) MichalN: Basically it should still be done, but only
conditionally if MSIs aren't used. Otherwise if the driver loads on a
system which for whatever reason doesn't use MSIs for xHCI, it'll
cause an interrupt storm.
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