Issue
According to Intel's document, FEE0 0000H
to FEE0 03F0H
is the range of local APIC. However, examining it from QEMU console, I can only find the value 0 there:
(qemu) x/128b 0xfee00000
fee00000: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00008: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00010: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00018: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00020: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00028: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00030: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00038: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00040: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00048: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00050: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00058: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00060: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00068: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00070: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
fee00078: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
I checked 128 bytes to see if anything different than 0. At least, FEE0 0030H
should contain a different value since it is where APIC version is in.
Paging is disabled, so there's no virtual memory; just physical:
(qemu) info mem
PG disabled
But, info lapic
command reports actual initial values:
(qemu) info lapic
dumping local APIC state for CPU 0
LVT0 0x00008700 active-hi level ExtINT (vec 0)
LVT1 0x00008400 active-hi level NMI
LVTPC 0x00010000 active-hi edge masked Fixed (vec 0)
LVTERR 0x00010000 active-hi edge masked Fixed (vec 0)
LVTTHMR 0x00010000 active-hi edge masked Fixed (vec 0)
LVTT 0x00030010 active-hi edge masked periodic Fixed (vec 16)
Timer DCR=0x2 (divide by 8) initial_count = 4096
SPIV 0x000001ff APIC enabled, focus=off, spurious vec 255
ICR 0x000c4610 physical edge assert all
ICR2 0x00000000
ESR 0x00000000
ISR (none)
IRR (none)
So, where are these values?
Solution
The device is emulated. When a vCPU reads or writes to those addresses, they are handled by the emulated APIC. When GDB/the QEMU console does, it probably doesn't get routed through that emulator (emulated reads can change state). Likely, gdb/the QEMU console just gets routed to QEMU's representation of RAM, which doesn't include the APIC.
Answered By - ruthafjord
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