On 01.07.14 16:58, Scott Wood wrote:
On Tue, 2014-07-01 at 08:23 +0200, Alexander Graf wrote:
Am 30.06.2014 um 22:25 schrieb Scott Wood <scottwood@xxxxxxxxxxxxx>:
On Sun, 2014-06-29 at 23:38 -0500, Bhushan Bharat-R65777 wrote:
-----Original Message-----
From: Wood Scott-B07421
Sent: Friday, June 27, 2014 11:53 PM
To: Bhushan Bharat-R65777
Cc: agraf@xxxxxxx; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
Subject: Re: [PATCH 2/2] KVM : powerpc/booke: Allow debug interrupt injection to
guest
On Fri, 2014-06-27 at 11:55 +0530, Bharat Bhushan wrote:
- /* Force enable debug interrupts when user space wants to debug */
- if (vcpu->guest_debug) {
+ /*
+ * Force enable debug interrupts when user space wants to debug
+ * and there is no debug interrupt pending for guest to handle.
+ */
+ if (vcpu->guest_debug && !kvmppc_core_pending_debug(vcpu)) {
Are you trying to allow the guest to be simultaneously debugged by itself and by
host userspace? How does this work?
Not actually, Currently we are not partitioning debug resources between
host userspace and guest. In fact we do not emulate debug registers for
guest. But we want host userspace to pass the interrupt to guest if it
is not able to handle.
I don't understand the logic here. A debug interrupt should be injected
when the programming model in the guest says that a debug interrupt
should happen. How can that occur currently? If the guest didn't set
up the debug registers and QEMU still can't handle the debug interrupt,
that's a bug in QEMU (or KVM, or the hardware...). Injecting the
interrupt into the guest just adds another bug on top of that.
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. 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.
The second problem I see is that we're disabling use cases for the sake
of correctness. When I use gdbstub, I usually don't want anything in the
way of debugging something - like if I want to debug the guest debugging
itself for example.
So overall, I think the x86 approach is a nice middle ground between
usability, performance and functionality.
one reg?
We are using SREGS but if required we can use one_reg.
I thought we were preferring one reg over sregs for new functionality.
I'm personally torn on this one. The problem here is that the sregs
fields and values are already reserved. For anything we don't have an
API for yet, yes, one_reg only. IIUC we have the API here, but were
lacking the implementation.
From a QEMU perspective, are we going to want to update this at the same
time as everything else, or would jumping through sregs hoops be a
nuisance? Is the long term goal to have one reg support for everything?
Because we need to maintain backwards compatibility, I don't think we'll
actually have any benefit from one reg support for everything.
I think we're in a path that is slow enough already to not worry about
performance. I think we're good to just call the sregs setter on demand
when we want to change single registers.
Alex
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