On 24/11/2014 15:52, Christoffer Dall wrote: > > We had a lenghty IRC discussion on this, for the curious, go read it > here: > http://irclogs.linaro.org/2014/11/24/%23kvm-arm.html
Some notes... > chazy but saying that you want to design an ABI that potentially exposes > fewer debug registers to gdbstubs than your hardware supports and breaks > gdbstub support if you happen to support emulation of a cpu with more debug > registers in the future, because you want to re-use ONE_REG is not a great > approach either > > agraf chazy: but you wouldn't even remotely think of doing it, no? > > agraf chazy: I'm saying that QEMU shouldn't have access to the excess > registers > > chazy agraf: I wouldn’t even remotely think of doing nested virtualization, > but people have > > agraf chazy: QEMU spawns an A57, it gets 8 debug registers > > chazy agraf: I understand, but I don’t agree with your rationale > > agraf chazy: not "QEMU spawns an A57 on an A67, so it gets 8 real and 8 > hidden debug registers that it also needs to be aware of mappings of" I disagree with Alex. QEMU can use the 16 debug registers as it wishes, since it sets them with KVM_SET_GUEST_DEBUG. The guest's eight can be mapped to the last 8 if those are the required semantics for the architecture. Just exit to QEMU if you do a debug register write while gdbstub debugging is active; QEMU gets the contents of the 8 VCPU-visible registers with ONE_REG (equivalent of x86 GET_DEBUGREGS); calls KVM_SET_GUEST_DEBUG to reflect them in the 16-register state; and restarts. It works as long as you can trap both reads and writes. > chazy ajb-linaro: don’t migrate a guest that is getting debugged is > the answer to that one I believe > > ajb-linaro chazy: at least with the overlay approach that happens > automatically > > ajb-linaro the overlay isn't migrated and the new host isn't calling > SET_GUEST_DEBUG so whatever the debug registers where before are restorerd Right. The destination will lose the hardware breakpoints/watchpoints because it starts the guest with KVM_SET_GUEST_DEBUG. Apart from losing them, everything should work fine. The dest sets the 8 VCPU-visible registers with ONE_REG, the hypervisor reflects them in a subset of the 16 physical registers, and "that's it". Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
