Drunken keyboard in go32v2 programs

Bug #1574246 reported by felix on 2016-04-24
This bug affects 1 person
Affects Status Importance Assigned to Milestone

Bug Description

QEMU 2.5.0, SeaBIOS 1.9.1; I've been noticing this bug for quite a while, though.

Steps to reproduce:

# Create a VM image, install DOS in it (doesn't matter which) and launch it.
# Launch a "bare DOS" DPMI host (not an operating system) in it; I tested with CWSDPMI and HDPMI32.
# Run a go32v2 program which reads keyboard input (say, the Lua interpreter: <http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.1/repos/devel/lua.zip>; the Free Pascal IDE will also do; on the other hand, DOS/4GW programs seem unaffected).
# Quickly type in something random (e.g. alternate between hitting "p" and "q"), then optionally move the cursor left and right.
# Observe how some keystrokes are missed, and some are caught twice.

The issue does NOT arise:
* on bare metal DOS,
* in VirtualBox,
* in Bochs with stock Plex86 BIOS,
* in Bochs with SeaBIOS,
* in DOSEMU,
* in DOSBox,
* in QEMU when the DPMI host is Windows 3.11/9x
so at this point I'm reasonably sure that it's the fault of either QEMU or SeaBIOS, and probably the former. The issue arises regardless of whether KVM is enabled.

felix (felix.von.s) wrote :
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I compiled the latest git snapshot (tag: v2.6.0-rc4, calling itself 2.5.94; with GTK frontend) and could only half-reproduce the bug; keys do not longer "jam", but arrow keys are still captured twice. I woudn't make much of that difference; this bug seems very timing-sensitive. It could be that the GTK frontend adds sufficient latency to the interface to avoid triggering it.

I enabled some debugging switches and recompiled both QEMU and the latest git snapshot of SeaBIOS (1.9.0-127; commit c8e105a4d5e52e8e7539ab1f2cd07ebe0ae9033a). This is what I got when running programs affected by this bug:

(key press)
[qemu ] ps2_queue(0xe0)
[qemu ] ps2_queue(0x4d)
[qemu ] KBD: kbd: read data=0xe0 ← (***)
[seabios] handle_09
[qemu ] KBD: kbd: read status=0x1d
[qemu ] KBD: kbd: read data=0x4d
[seabios] i8042_command cmd=ae
[seabios] i8042_wait_write
[qemu ] KBD: kbd: read status=0x1c
[qemu ] KBD: kbd: write cmd=0xae
[qemu ] KBD: kbd: read data=0x4d ← (***)
[seabios] handle_09
[qemu ] KBD: kbd: read status=0x1c
[qemu ] KBD: kbd: read data=0x4d
[seabios] i8042_command cmd=ae
[seabios] i8042_wait_write
[qemu ] KBD: kbd: read status=0x1c
[qemu ] KBD: kbd: write cmd=0xae

Reads marked (***) do not appear when running unaffected programs. So it appears something is making reads from the keyboard controller before SeaBIOS has a chance to put scancodes in the ring buffer. And indeed: DJGPP libc installs a custom IRQ1 handler which does it to detect whether it should raise SIGINT in response to Ctrl+C; it can be found in the file src/libc/go32/exceptn.S in the djcrx package[1]. Free Pascal incorporates this handler into its RTL with almost no changes; it's found in rtl/go32v2/exceptn.as[2]. I also noticed I can reproduce this bug with Borland Pascal 7; lo and behold, the Turbo Vision library does something similar.

SeaBIOS gets extra confused when I send some mouse events to QEMU (grab the mouse, move it around, ungrab); it reacts with a "ps2 keyboard irq but found mouse data?!" message and refuses to put keys in the ring buffer until the queue of mouse events becomes empty.

That's the culprit, I think. It also explains why the bug doesn't appear under Windows; because port 0x60 reads from DPMI are simulated, they don't correspond to actual port 0x60 reads in the guest. I don't know what the fix ought be; documentation about the i8042 that I found is unclear about what real hardware does in this case.

If I'm reading the code correctly, DOSEMU[3] (also the DOSEMU2 fork[4]), DOSBox[5], Bochs[6] and VirtualBox[7] keep one value to be read from 0x60 at until the next interrupt, avoiding the issue.

[1] <http://www.delorie.com/bin/cvsweb.cgi/djgpp/src/libc/go32/exceptn.S?rev=1.7>; function ___djgpp_kbd_hdlr
[2] <http://svn.freepascal.org/cgi-bin/viewvc.cgi/trunk/rtl/go32v2/exceptn.as?revision=8210&view=co>; function ___djgpp_kbd_hdlr
[3] <https://sourceforge.net/p/dosemu/code/ci/8897222f8830e0bd2769935f611a0e5c3271bcb9/tree/src/plugin/kbd_unicode/serv_8042.c>
[4] <https://github.com/stsp/dosemu2/blob/69419c7a5430f0109f9df45b179d9b223b075550/src/plugin/kbd_unicode/serv_8042.c>
[5] <https://sourceforge.net/p/dosbox/code-0/3961/tr...


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