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[Bug 1693050] Re: xhci HCIVERSION register read emulation incorrectly ha
[Bug 1693050] Re: xhci HCIVERSION register read emulation incorrectly handled
Wed, 25 Mar 2020 20:50:52 -0000
According to , this is still an issue today.
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xhci HCIVERSION register read emulation incorrectly handled
Status in QEMU:
We had an illumos user trying to run illumos in QEMU 2.9.0 with the
qemu-xhci device enabled. Note, that while this was discovered against
QEMU 2.9.0, from my current read of the HEAD, it is still present. The
illumos bug at https://www.illumos.org/issues/8173 has additional
information on how the user invoked qemu. While investigating the
problem we found that the illumos driver was reading a version of
0x0000 when reading the HCIVERSION register from qemu.
In the illumos driver we're performing a 16-bit read of the version
register at offset 0x2. From looking around at other OSes, while Linux
performs a 4 byte read at offset 0x0 and masks out the version, others
that care about the version are doing a two byte read, though not all
actually act on the version and some just discard the information.
The user who hit this was able to enable tracing (note the tracing
file is attached to the illumos bug linked previously) and we hit the
unimplemented register read with offset 0x2 at
xhci.c;hb=HEAD#l2960. The xhci register specifies today that its
allowed for users to do 1-4 byte reads; however, that it implements
only four byte reads in its implementation
xhci.c;hb=HEAD#l3333). Hence why when we read the HCIVERSION register
at offset 0x2, it isn't handled in xhci_cap_read() which then returns
From digging into this, I think that we're coming into
memory_region_dispatch_read() and then memory_region_dispatch_read1().
However, I don't see anything in either the general memory region
logic or in the xhci_cap_read() function that would deal with
adjusting the offset that we're reading at and then masking it off
again. While the access_with_adjusted_size() attempts to deal with
this, it never adjusts the hardware address that's passed in to be a
multiple of the implementation defined offset that I can see. I
suspect that the FIXME at line 582
the implementation in the xhci code is the crux of the problem.
For the time being we're working around this in the illumos driver,
but I wanted to point this out such that it might be helpful for other
systems which are assuming that they can do the two byte read like on
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Paul Menzel <=