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

From: Alexander Graf <[email-masked]>
Date: Tue, 17 Jun 2014 13:31:22 +0200

On 17.06.14 13:20, Madhavan Srinivasan wrote:
> On Tuesday 17 June 2014 03:13 PM, Alexander Graf wrote:
>> 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
>   ^^^ Where is this taken care. I am still trying to understand. Kindly
> can you explain or point to the code. Will help.

We tell the guest via VPA about its steal time which includes QEMU time.

Alex