On Tue, Mar 16, 2021, Maxim Levitsky wrote: > This change greatly helps with two issues: > > * Resuming from a breakpoint is much more reliable. > > When resuming execution from a breakpoint, with interrupts enabled, more often > than not, KVM would inject an interrupt and make the CPU jump immediately to > the interrupt handler and eventually return to the breakpoint, to trigger it > again. > > From the user point of view it looks like the CPU never executed a > single instruction and in some cases that can even prevent forward progress, > for example, when the breakpoint is placed by an automated script > (e.g lx-symbols), which does something in response to the breakpoint and then > continues the guest automatically. > If the script execution takes enough time for another interrupt to arrive, > the guest will be stuck on the same breakpoint RIP forever. > > * Normal single stepping is much more predictable, since it won't land the > debugger into an interrupt handler, so it is much more usable. > > (If entry to an interrupt handler is desired, the user can still place a > breakpoint at it and resume the guest, which won't activate this workaround > and let the gdb still stop at the interrupt handler) > > Since this change is only active when guest is debugged, it won't affect > KVM running normal 'production' VMs. > > > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx> > Tested-by: Stefano Garzarella <sgarzare@xxxxxxxxxx> > --- > arch/x86/kvm/x86.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index a9d95f90a0487..b75d990fcf12b 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8458,6 +8458,12 @@ static void inject_pending_event(struct kvm_vcpu *vcpu, bool *req_immediate_exit > can_inject = false; > } > > + /* > + * Don't inject interrupts while single stepping to make guest debug easier > + */ > + if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) > + return; Is this something userspace can deal with? E.g. disable IRQs and/or set NMI blocking at the start of single-stepping, unwind at the end? Deviating this far from architectural behavior will end in tears at some point. > + > /* > * Finally, inject interrupt events. If an event cannot be injected > * due to architectural conditions (e.g. IF=0) a window-open exit > -- > 2.26.2 >
