State of debug register emulation
Hi,
looks like we are getting better and better here in hitting yet unsupported corner-case features of KVM :). This time our guest fiddles with hardware debugging registers, but quickly gets unhappy as they do not yet have the expected effect.
Joerg, I found you SVM-related patch series in the archive which does not seem to have raised much responses. Is this general direction OK? Does it allow self-debugging of guests? But how are conflicts resolved if both guest and host need the physical registers (host debugging the guest which is debugging itself)?
I would try to dig into the VMX side if the general architecture is -mostly- clear. [ Sorry, Joerg, someone put the latter type of HW on my desk :->. Hope I can once check our stuff against SVM as well! ]
Thanks,
Jan
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Quoted 6 lines · Jan Kiszka
Hi,
looks like we are getting better and better here in hitting yet unsupported corner-case features of KVM :). This time our guest fiddles with hardware debugging registers, but quickly gets unhappy as they do not yet have the expected effect.
KVM is mostly tested with guests that run with paging. So a 16 bit protected mode guest is not tested very well :)
Quoted 5 lines
Joerg, I found you SVM-related patch series in the archive which does not seem to have raised much responses. Is this general direction OK? Does it allow self-debugging of guests? But how are conflicts resolved if both guest and host need the physical registers (host debugging the guest which is debugging itself)?
I sent a patchset in the past to enable guest debugging for SVM which means debugging the guest from outside using gdb. But I was not able to test these patches because the userspace side of guest debugging is broken in the kvm-qemu.
Debugging in the guest should work without problems. The debug registers are switched between guest and host if the guest uses them. So there should be no problems when the guest and the host using the debug registers.
Quoted 3 lines
I would try to dig into the VMX side if the general architecture is -mostly- clear. [ Sorry, Joerg, someone put the latter type of HW on my desk :->. Hope I can once check our stuff against SVM as well! ]
With some debug output from SVM I can better help to debug your problems ;-)
Joerg
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http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaoneQuoted 10 lines · Joerg Roedel
Quoted 6 lines · Jan Kiszka
Hi,
looks like we are getting better and better here in hitting yet unsupported corner-case features of KVM :). This time our guest fiddles with hardware debugging registers, but quickly gets unhappy as they do not yet have the expected effect.
KVM is mostly tested with guests that run with paging. So a 16 bit protected mode guest is not tested very well :)
Yes, we know (we also had a bit fun with stock QEMU in corner cases). But that may change now... :)
Quoted 7 lines
Quoted 6 lines
Joerg, I found you SVM-related patch series in the archive which does not seem to have raised much responses. Is this general direction OK? Does it allow self-debugging of guests? But how are conflicts resolved if both guest and host need the physical registers (host debugging the guest which is debugging itself)?
Quoted 8 lines
I sent a patchset in the past to enable guest debugging for SVM which means debugging the guest from outside using gdb. But I was not able to test these patches because the userspace side of guest debugging is broken in the kvm-qemu.
Debugging in the guest should work without problems. The debug registers are switched between guest and host if the guest uses them. So there should be no problems when the guest and the host using the debug registers.
I'm currently digging my way through the current VMX code, but I cannot confirm this. Not sure what SVM does, but as far as I understood the VMX side, only DR7 is saved/restored in hardware. The rest is KVM's job. Unfortunately the access to the real debug registers only happens "if (vcpu->guest_debug.enabled)". And as all DR accesses of the guest are trapped, but the desired transfers to/from guest registers are nops, this cannot work yet, at least on VMX.
This still leaves me with the question how to handle the case when the host sets and arms some debug registers to debug the guest and the latter does the same to debug itself. Guest access will be trapped, OK, but KVM will then have to decide which value should actually be transfered into the registers. Hmm, does SVM virtualizes all debug registers, leaving the real ones to the host?
Quoted 7 lines
Quoted 4 lines
I would try to dig into the VMX side if the general architecture is -mostly- clear. [ Sorry, Joerg, someone put the latter type of HW on my desk :->. Hope I can once check our stuff against SVM as well! ]
With some debug output from SVM I can better help to debug your problems ;-)
I'm sure :). But I guess this topic has a few common aspects to be solved, too. So we may ideally end up with a single series of debug-enabling patches for KVM (maybe even fixing userland - we are not totally unfamiliar with the gdbstub here).
Jan
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Quoted 7 lines · Jan Kiszka
This still leaves me with the question how to handle the case when the host sets and arms some debug registers to debug the guest and the latter does the same to debug itself. Guest access will be trapped, OK, but KVM will then have to decide which value should actually be transfered into the registers. Hmm, does SVM virtualizes all debug registers, leaving the real ones to the host?
There's no way this can work. There are still only four debug registers, and the guest and host together can ask for eight different addresses. It is theoretically doable by hiding all mappings to pages that are debug targets, but it would probably double the kvm code size.
A good short-term compomise is to abort if the guest starts using enabling a debug address register. A better solution might be to place host debug addresses into unused guest debug registers, so that as long as nr_guest_debug + nr_host_debug <= 4, we can still proceed.
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Quoted 14 lines · Avi Kivity
Quoted 7 lines · Jan Kiszka
This still leaves me with the question how to handle the case when the host sets and arms some debug registers to debug the guest and the latter does the same to debug itself. Guest access will be trapped, OK, but KVM will then have to decide which value should actually be transfered into the registers. Hmm, does SVM virtualizes all debug registers, leaving the real ones to the host?
There's no way this can work. There are still only four debug registers, and the guest and host together can ask for eight different addresses. It is theoretically doable by hiding all mappings to pages that are debug targets, but it would probably double the kvm code size.
Quoted 4 lines
A good short-term compomise is to abort if the guest starts using enabling a debug address register. A better solution might be to place host debug addresses into unused guest debug registers, so that as long as
nr_guest_debug+nr_host_debug<= 4, we can still proceed.
I tried the latter, but we cannot cleanly share DR7 between both users. Thus I'm now going for a prioritized approach: debug register will stop to have any effect for the guest as soon as the host starts to use them. That's far simpler the implement and also easier to understand for the user.
A bit work remains, though, to clean up and enhance the DRx support in KVM. And to test the changes (will contact you, Joerg, regarding SVM tests). Stay tuned.
Jan
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Quoted 17 lines · Jan Kiszka
Quoted 15 lines · Avi Kivity
Quoted 8 lines · Jan Kiszka
This still leaves me with the question how to handle the case when the host sets and arms some debug registers to debug the guest and the latter does the same to debug itself. Guest access will be trapped, OK, but KVM will then have to decide which value should actually be transfered into the registers. Hmm, does SVM virtualizes all debug registers, leaving the real ones to the host?
There's no way this can work. There are still only four debug registers, and the guest and host together can ask for eight different addresses. It is theoretically doable by hiding all mappings to pages that are debug targets, but it would probably double the kvm code size.
Quoted 8 lines
Quoted 5 lines
A good short-term compomise is to abort if the guest starts using enabling a debug address register. A better solution might be to place host debug addresses into unused guest debug registers, so that as long as
nr_guest_debug+nr_host_debug<= 4, we can still proceed.I tried the latter, but we cannot cleanly share DR7 between both users.
I actually think we can, but...
Quoted 5 lines
Thus I'm now going for a prioritized approach: debug register will stop to have any effect for the guest as soon as the host starts to use them. That's far simpler the implement and also easier to understand for the user.
Agreed, having a simple model is preferred here, both from the user's point of view and from a code complexity point of view. If you're debugging a debugger use plain qemu.
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