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. > amount of real time, no? > > > Alex > Thanks for review. Regards Maddy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
