Re: [Qemu-devel] Re: POST failure (loop) with isapc and seabios

[Gleb Natapov]

On Wed, Nov 25, 2009 at 12:27:06AM +0100, Sebastian Herbszt wrote:
> Gleb Natapov wrote:
> >On Mon, Nov 23, 2009 at 10:30:56PM +0100, Sebastian Herbszt wrote:
> >>Gleb Natapov wrote:
> >>>On Mon, Nov 23, 2009 at 08:19:54PM +0100, Sebastian Herbszt wrote:
> >>>>Gleb Natapov wrote:
> >>>>>On Sun, Nov 22, 2009 at 09:01:45PM +0100, Sebastian Herbszt wrote:
> >>>>>>Gleb Natapov wrote:
> >>>>>>>On Sun, Nov 22, 2009 at 04:31:24PM +0100, Sebastian Herbszt wrote:
> >>>>>>>>
> >>>>>>>>Bad things could happen if someone modifies the BIOS because it's 
> >>>>>>>>unprotected
> >>>>>>>>(e.g. VM crash).
> >>>>>>>>
> >>>>>>>BIOS is reloaded during VM reset.
> >>>>>>
> >>>>>>The BIOS is not reloaded - tested with "reboot" on Linux and 
> >>>>>>system_reset in monitor.
> >>>>>>
> >>>>>Looks like a bug. Tested with latest QEMU version I assume?
> >>>>
> >>>>Tested with v0.11.0-rc0-1677-gf165b53. Where do you suspect a bug? In the 
> >>>>behaviour
> >>>>on "reboot" or system_reset? I think it depends wheter it's a software or 
> >>>>hardware reset.
> >>>>
> >>>The bug is that BIOS rom is not reloaded on hardware reset (what do you
> >>>call software reset?). Can you retest with latest git?
> >>
> >>Same with v0.11.0-rc0-1699-gb0293e5.
> >>Bochs has the notion of BX_RESET_SOFTWARE and BX_RESET_HARDWARE. Those
> >>two describe the source of the reset and the hardware, in most cases just 
> >>the cpu, is reset
> >>differently depending on the source (e.g. hardware reset / power on vs. 
> >>INIT#). I think this
> >>behaviour is also described in the 440fx spec (chapter "4.5.3 SYSTEM 
> >>RESET").
> >>
> >>If the monitor command "system_reset" is supposed to be the equivalent of 
> >>pushing the reset
> >>button of the box, then the bios should be reloaded (i think, not entirely 
> >>sure what the pin is
> >>wired to). But the bios should not be reloaded if the OS just jumps to the 
> >>bios reset code.
> >>
> >Correct, but modern OSes never jump to the bios reset code. They use
> >ACPI reset, kbd reset or triple fault to do reset. All of those generate
> >exactly the same kind of reset as "system_reset" monitor command.
> 
> With the 440FX you get two sources of a hard reset - power on and i/o write 
> to reset control register.
> Tho the PMC should not be used to generate a hard reset if used with a PIIX3. 
> You get a soft reset on
> CPU shutdown bus cycle, i/o write to keyboard controller, i/o write to port 
> 92h and i/o write to reset
> control register. The latter can be used for hard and soft resets. During a 
> hard reset the PMC sets its
> configuration registers to default values. The PAM registers are set to 0 and 
> the BIOS is accessed thru
> rom and not shadow ram. On a triple fault the CPU generates the shutdown bus 
> cycle and since the PMC
> responds to it with a soft reset the BIOS is not reloaded (?). I might have 
> missed something in some
> specification tho.
> 
Nice sum up. Thanks. When I said "all of those generate exactly the same
kind of reset as "system_reset" monitor command" I meant in QEMU, not on
actual HW. QEMU has no notion of different type of resets, though it
should.

--
                        Gleb.