----- Original Message ----- > On Sun, Oct 2, 2011 at 4:56 PM, Avi Kivity <a...@redhat.com> wrote: > > On 10/01/2011 10:31 AM, Blue Swirl wrote: > >> > >> Therefore it is incorrect to perform any qemu_irq activities > >> during > >> reset (also VM restore like the original example), don't you > >> agree? > > > > It is not incorrect. Real hardware updates outputs on RESET > > assertion, and > > real hardware deals with devices entering reset at different times > > (due to > > signal propagation delay or slow devices). > > Yes, but on real hardware, during the propagation of any effects, the > reset line is held asserted for millions of clock cycles in order to > stabilize the machine.
But nothing can happen in these cycles, since qemu emulates everything that happens in the on the edge, and any timers will be cancelled. > > >> If we continued to reset all the devices (call the reset handlers > >> multiple times), eventually machine state should stabilize > >> (equivalent > >> of real HW with nice long reset pulses), but on QEMU the reset > >> event > >> is infinitely short so we have to be more careful. > > > > calling qemu_irq_pulse(reset) simulates a reset signal of any > > length (since > > nothing happens between the two edges). > > Not really. The first edge could trigger the reset (but don't sample > inputs) phase, this would be equal to forcefully stabilizing any > device internal state. The second would release the device inputs, > but > that's also a source of problems. Can you elaborate on the problems? > > >> Actually I don't think that even a two-phase reset with qemu_irq > >> or > >> Pin activity on the second phase would produce correct results in > >> every obscure case. Though this may be detectable since the start > >> state would be known. > > > > The output signals have to stabilize before the second edge of the > > reset > > signal. > > They can't since the devices' inputs are ignored at that phase. > A real device also ignores inputs during reset (or if it doesn't, we can just emulate that). I would really like to see a concrete example we can discuss.
