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[Xen-devel] [BUG] x86/hvm/vioapic: 8-Bit IOREGSEL write does not work

I've seen this problem with Xen 4.6.5 from the Xubuntu 16.04
distribution and from a quick look over the current vioapic code it
seems to be still present.

From the IOAPIC datasheet [1]: "To reference an IOAPIC register, a byte
memory write that the PIIX3 decodes for the IOAPIC loads the IOREGSEL
Register with an 8-bit value that specifies the IOAPIC register (address
offset in Table 3.2) to be accessed."

But the 'vioapic_write()' function does not consider the 'length' argument
and always copies 4 Bytes from the unsigned long 'val' argument into the
virtual 32-Bit IOREGSEL register. In the error case I've seen, 'length'
was 1 and 'val' was 0xffff8300bb0cf801 and the IOAPIC version register
with address offset 0x01 was not read correctly.


[1] https://pdos.csail.mit.edu/6.828/2016/readings/ia32/ioapic.pdf

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