Re: [Qemu-devel] Regression with floppy drive controller

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing Eduardo, Paolo.

On 8/20/19 3:38 PM, Philippe Mathieu-Daudé wrote:
> On 8/20/19 3:12 PM, John Snow wrote:
>> On 8/20/19 6:25 AM, Philippe Mathieu-Daudé wrote:
>>> [cross posting QEMU & SeaBIOS]
>>>
>>> Hello,
>>>
>>> I'v been looking at a QEMU bug report [1] which bisection resulted in a
>>> SeaBIOS commit:
>>>
>>> 4a6dbcea3e412fe12effa2f812f50dd7eae90955 is the first bad commit
>>> commit 4a6dbcea3e412fe12effa2f812f50dd7eae90955
>>> Author: Nikolay Nikolov <nickysn@users.sourceforge.net>
>>> Date:   Sun Feb 4 17:27:01 2018 +0200
>>>
>>>     floppy: Use timer_check() in floppy_wait_irq()
>>>
>>>     Use timer_check() instead of using floppy_motor_counter in BDA for the
>>>     timeout check in floppy_wait_irq().
>>>
>>>     The problem with using floppy_motor_counter was that, after it reaches
>>>     0, it immediately stops the floppy motors, which is not what is
>>>     supposed to happen on real hardware. Instead, after a timeout (like in
>>>     the end of every floppy operation, regardless of the result - success,
>>>     timeout or error), the floppy motors must be kept spinning for
>>>     additional 2 seconds (the FLOPPY_MOTOR_TICKS). So, now the
>>>     floppy_motor_counter is initialized to 255 (the max value) in the
>>>     beginning of the floppy operation. For IRQ timeouts, a different
>>>     timeout is used, specified by the new FLOPPY_IRQ_TIMEOUT constant
>>>     (currently set to 5 seconds - a fairly conservative value, but should
>>>     work reliably on most floppies).
>>>
>>>     After the floppy operation, floppy_drive_pio() resets the
>>>     floppy_motor_counter to 2 seconds (FLOPPY_MOTOR_TICKS).
>>>
>>>     This is also consistent with what other PC BIOSes do.
>>>
>>>
>>> This commit improve behavior with real hardware, so maybe QEMU is not
>>> modelling something or modelling it incorrectly?
> [...]
>>
>> Well, that's unfortunate.
>>
>> What version of QEMU shipped the SeaBIOS that caused the regression?
> 
> See https://bugs.launchpad.net/qemu/+bug/1840719/comments/3
> 
> QEMU commit 0b8f74488e, slighly before QEMU v3.1.0
> (previous tag is v3.0.0).
> 
> But you can use v4.1.0 too, simply change the SeaBIOS bios.bin, i.e.:
> 
>   qemu$ git checkout v4.1.0
> 
>   qemu$ (cd roms/seabios && git checkout 4a6dbcea3e4~) && \
>         make -C roms bios
> 
> Now pc-bios/bios.bin is built using the last commit working,
> 
>   qemu$ (cd roms/seabios && git checkout 4a6dbcea3e4) && \
>         make -C roms bios
> 
> And you can reproduce the error.

Back from here.

So the SeaBIOS patch is:

diff --git a/src/hw/floppy.c b/src/hw/floppy.c
index 77dbade..3012b3a 100644
--- a/src/hw/floppy.c
+++ b/src/hw/floppy.c
@@ -34,6 +34,7 @@
 #define FLOPPY_GAPLEN 0x1B
 #define FLOPPY_FORMAT_GAPLEN 0x6c
 #define FLOPPY_PIO_TIMEOUT 1000
+#define FLOPPY_IRQ_TIMEOUT 5000

 #define FLOPPY_DOR_MOTOR_D     0x80 // Set to turn drive 3's motor ON
 #define FLOPPY_DOR_MOTOR_C     0x40 // Set to turn drive 2's motor ON
@@ -221,8 +222,9 @@ floppy_wait_irq(void)
 {
     u8 frs = GET_BDA(floppy_recalibration_status);
     SET_BDA(floppy_recalibration_status, frs & ~FRS_IRQ);
+    u32 end = timer_calc(FLOPPY_IRQ_TIMEOUT);
     for (;;) {
-        if (!GET_BDA(floppy_motor_counter)) {
+        if (timer_check(end)) {
             warn_timeout();
             floppy_disable_controller();
             return DISK_RET_ETIMEOUT;

timer_calc() unit is milliseconds, so this patch should wait upto
5seconds before failing, and it seems the timeout is not used at all.

SeaBIOS timer.c:

// Return the TSC value that is 'msecs' time in the future.
u32
timer_calc(u32 msecs)
{
    return timer_read() + (GET_GLOBAL(TimerKHz) * msecs);
}

static u32
timer_read(void)
{
    u16 port = GET_GLOBAL(TimerPort);
    if (CONFIG_TSC_TIMER && !port)
        // Read from CPU TSC
        return rdtscll() >> GET_GLOBAL(ShiftTSC);
    if (CONFIG_PMTIMER && port != PORT_PIT_COUNTER0)
        // Read from PMTIMER
        return timer_adjust_bits(inl(port), 0xffffff);
    // Read from PIT.
    outb(PM_SEL_READBACK | PM_READ_VALUE | PM_READ_COUNTER0, PORT_PIT_MODE);
    u16 v = inb(PORT_PIT_COUNTER0) | (inb(PORT_PIT_COUNTER0) << 8);
    return timer_adjust_bits(v, 0xffff);
}

Using the default QEMU config, we build SeaBIOS to use the TSC timer:

builds/seabios-128k/.config:CONFIG_TSC_TIMER=y
builds/seabios-256k/.config:CONFIG_TSC_TIMER=y

$ qemu-system-i386 -M isapc -cpu 486 \
  -fda Windows\ 98\ Second\ Edition\ Boot.img \
  -chardev stdio,id=seabios \
  -device isa-debugcon,iobase=0x402,chardev=seabios
Booting from Floppy...
Floppy_drive_recal 0
Floppy_enable_controller
WARNING - Timeout at floppy_wait_irq:228!
Floppy_disable_controller
Floppy_enable_controller
WARNING - Timeout at floppy_wait_irq:228!
Floppy_disable_controller
Boot failed: could not read the boot disk

Now enabling the TSC feature:

$ qemu-system-i386 -M isapc -cpu 486,tsc \
  -fda Windows\ 98\ Second\ Edition\ Boot.img \
  -chardev stdio,id=seabios \
  -device isa-debugcon,iobase=0x402,chardev=seabios
Booting from Floppy...
Floppy_drive_recal 0
Floppy_enable_controller
Floppy_media_sense on drive 0 found rate 0
Booting from 0000:7c00
Floppy_disable_controller
Floppy_enable_controller
Floppy_drive_recal 0
Floppy_media_sense on drive 0 found rate 0

Do we need a cpu with TSC support to run SeaBIOS?

So we should use '-cpu Conroe' or '-cpu core2duo' minimum?