Re: [PATCH 2/2] KVM : powerpc/booke: Allow debug interrupt injection to guest

[Scott Wood <scottwood@freescale.com>]

On Tue, 2014-07-01 at 18:22 +0200, Alexander Graf wrote:
> On 01.07.14 17:35, Scott Wood wrote:
> > On Tue, 2014-07-01 at 17:04 +0200, Alexander Graf wrote:
> >> On 01.07.14 16:58, Scott Wood wrote:
> >>> On Tue, 2014-07-01 at 08:23 +0200, Alexander Graf wrote:
> >>>> I don't think QEMU should be aware of these limitations.
> >>> OK, but we should at least have some idea of how the whole thing is
> >>> supposed to work, in order to determine if this is the correct behavior
> >>> for QEMU.  I thought the model was that debug resources are either owned
> >>> by QEMU or by the guest, and in the latter case, QEMU would never see
> >>> the debug exception to begin with.
> >> That's bad for a number of reasons. For starters it's different from how
> >> x86 handles debug registers - and I hate to be different just for the
> >> sake of being different.
> > How does it work on x86?
> 
> It overwrites more-or-less random breakpoints with its own ones, but 
> leaves the others intact ;).

Are you talking about software breakpoints or management of hardware
debug registers?

> >> So if we do want to declare that debug registers are owned by either
> >> QEMU or the guest, we should change the semantics for all
> >> architectures.
> > If we want to say that ownership of the registers is shared, we need a
> > plan for how that would actually work.
> 
> I think you're overengineering here :). When do people actually use 
> gdbstub? Usually when they want to debug a broken guest. We can either
> 
>    * overengineer heavily and reduce the number of registers available 
> to the guest to always have spares
>    * overengineer a bit and turn off guest debugging completely when we 
> use gdbstub
>    * just leave as much alive as we can, hoping that it helps with the 
> debugging
> 
> Option 3 is what x86 does - and I think it's a reasonable approach. This 
> is not an interface that needs to be 100% consistent and bullet proof, 
> it's a best effort to enable you to debug as much as possible.

I'm not insisting on 100% -- just hoping for some explanation/discussion
about how it's intended to work for the cases where it can.

How will MSR[DE] and MSRP[DEP] be handled?

How would I go about telling QEMU/KVM that I don't want this shared
mode, because I don't want guest to interfere with the debugging I'm
trying to do from QEMU?

Will guest accesses to debug registers cause a userspace exit when
guest_debug is enabled?

> >> I think we're in a path that is slow enough already to not worry about
> >> performance.
> > It's not just about performance, but simplicity of use, and consistency
> > of API.
> >
> > Oh, and it looks like there already exist one reg definitions and
> > implementations for most of the debug registers.
> 
> For BookE? Where?

arch/powerpc/kvm/booke.c: KVM_REG_PPC_IACn, KVM_REG_PPC_DACn

-Scott