On 13/05/2019 19:22, Fabiano Rosas wrote: > Mark Cave-Ayland <mark.cave-ayland@xxxxxxxxxxxx> writes: > >> On 13/05/2019 07:01, Alexey Kardashevskiy wrote: >> >>> On 12/05/2019 00:08, Mark Cave-Ayland wrote: >>>> Hi all, >>>> >>>> Whilst trying to investigate some issues with MacOS under KVM PR I noticed that when >>>> setting software breakpoints the KVM VCPU would stop as requested, but QEMU's gdbstub >>>> would hang indefinitely. >>> >>> What are you trying to do exactly? Just breakpoints or single stepping? >>> Anyway, I am cc-ing Fabiano who is fixing single stepping and knows this >>> code well. >> >> I'm currently investigating why MacOS 9 fails to start up on KVM using a G4 Mac Mini, >> and my starting point is to do a side-by-side comparison with TCG which can boot MacOS 9. >> >> I discovered this issue setting a software breakpoint using QEMU's gdbstub and >> finding that whilst execution of the vCPU paused as expected, QEMU would hang because >> with run->debug.arch.status != 0 the gdbstub tries to handle it as a hardware >> breakpoint instead of a software breakpoint causing confusion. >> >> I've also tried using single-stepping which mostly works, however during OS startup >> as soon as I step over a mtsrr1 instruction, I lose the single-stepping and vCPU runs >> as normal. My suspicion here is that something in the emulation code is losing the >> MSR_SE bit, but I need to dig a bit deeper here. > > I would expect that a mtsrr1 followed by rfid would cause this sort of > behavior since MSR_SE is set/cleared at each guest entry/exit > (kvmppc_setup_debug and kvmppc_clear_debug functions) and whatever was > copied into SRR1 might not have MSR_SE set. Yes indeed, I can confirm that's the sequence which is causing the issue with single-stepping here: 0x0020f0a4: 8203001c lwz r16, 0x1c(r3) 0x0020f0a8: 7cba03a6 mtspr 0x1a, r5 0x0020f0ac: 38003000 li r0, 0x3000 0x0020f0b0: 7c1b03a6 mtspr 0x1b, r0 0x0020f0b4: 4c000064 rfi What would be the right way to handle this? Is it necessary to emulate rfi if single-step mode is enabled to ensure MSR_SE is still set? ATB, Mark.
