Re: [PATCH 2/3] KVM: x86: guest debug: don't inject interrupts while single stepping

[Sean Christopherson <seanjc@google.com>]

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@redhat.com>
> Tested-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  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
>