Hello everybody!
I've been experimenting with QEMU, VFIO and GPU passthrough, and, suddenly,
encountered a bug.
My VM(launched from the script [1]) starts booting fine, but then comes to
a complete stop with 100% load on one CPU.
To investigate the issue, I've connected GDB to qemu, and what I've seen
was very strange.[2]
There is some strange way to write zero to RAX, then we compare RAX with
RAX and do JE back to the write-zero part.
Using binary grep(bgrep or some other ways), I've found that op-code byte
sequence in OVMF-pure-efi.fd:
hexdump -C /usr/share/edk2.git/ovmf-x64/OVMF-pure-efi.fd | grep "74 f7 c9"
001cdaf0 90 48 8b 45 f8 48 85 c0 74 f7 c9 c3 55 48 89 e5 |.H.E.H..t...UH..|
8b 45 f8 is MOV -0x8 RAX, 74 f7 is JE -0x7
When changing JE to JNE on a live, hung VM via GDB - the VM continues to
boot fine without any noticeable anomalies.
VM doesn't hang when VFIO device is removed from the VM.
When that given device is connected directly to the hardware, Asus'
proprietary UEFI successfully boots and works.
I've got my OVMF binary from [3], which appears to be jenkins-based
automated build system. Build number doesn't seem to matter much, because I
have been experiencing this problem for 3-4 months, and yet nothing changed
much.
I'm using fedora 21 and QEMU emulator version 2.1.3 (qemu-2.1.3-2.fc21)
with kernel 3.18.7-200.fc21.x86_64.
Since i'm not very familiar with GDB and debugging UEFI/firmwares in
general, can someone please determine where is the actual problem, and what
does that weird assembly code really does?
I can provide the ROM file that I am using and any other needed information
to investigate the issue.
Links:
[1]: http://pastebin.com/cy7ZYvjc
[2]: http://pastebin.com/Q5Q9Fwgp
[3]: https://www.kraxel.org/repos/
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