Subject: Re: [PATCH] powerpc/kvm: support to handle sw breakpoint

From: Alexander Graf <[email-masked]>
Date: Tue, 17 Jun 2014 11:43:20 +0200

On 17.06.14 11:32, Benjamin Herrenschmidt wrote:
> On Tue, 2014-06-17 at 11:25 +0200, Alexander Graf wrote:
>> On 17.06.14 11:22, Benjamin Herrenschmidt wrote:
>>> On Tue, 2014-06-17 at 10:54 +0200, Alexander Graf wrote:
>>>> Also, why don't we use twi always or something else that actually is
>>>> defined as illegal instruction? I would like to see this shared with
>>>> book3s_32 PR.
>>> twi will be directed to the guest on HV no ? We want a real illegal
>>> because those go to the host (for potential emulation by the HV).
>> Ah, good point. I guess we need different one for PR and HV then to
>> ensure compatibility with older ISAs on PR.
> Well, we also need to be careful with what happens if a PR guest puts
> that instruction in, do that stop its HV guest/host ?
>
> What if it's done in userspace ? Do that stop the kernel ? :-)

The way SW breakpointing is handled is that when we see one, it gets 
deflected into user space. User space then has an array of breakpoints 
it configured itself. If the breakpoint is part of that list, it 
consumes it. If not, it injects a debug interrupt (program in this case) 
into the guest.

That way we can overlay that one instruction with as many layers as we 
like :). We only get a performance hit on execution of that instruction.

> Maddy, I haven't checked, does your patch ensure that we only ever stop
> if the instruction is at a recorded bkpt address ? It still means that a
> userspace process can practically DOS its kernel by issuing a lot of
> these causing a crapload of exits.

Only user space knows about its breakpoint addresses, so we have to 
deflect. However since time still ticks on, we only increase jitter of 
the guest. The process would still get scheduled away after the same 
amount of real time, no?

Alex