Gleb Natapov wrote: > On Sun, Feb 14, 2010 at 06:06:31PM +0100, Jan Kiszka wrote: >> Gleb Natapov wrote: >>> On Sun, Feb 14, 2010 at 05:37:39PM +0100, Jan Kiszka wrote: >>>> Gleb Natapov wrote: >>>>> On Sun, Feb 14, 2010 at 12:39:14PM +0100, Jan Kiszka wrote: >>>>>> Gleb Natapov wrote: >>>>>>> On Sun, Feb 14, 2010 at 11:47:58AM +0100, Jan Kiszka wrote: >>>>>>>> Gleb Natapov wrote: >>>>>>>>> On Sun, Feb 14, 2010 at 11:26:31AM +0100, Jan Kiszka wrote: >>>>>>>>>> Gleb Natapov wrote: >>>>>>>>>>> On Sat, Feb 13, 2010 at 10:31:12AM +0100, Jan Kiszka wrote: >>>>>>>>>>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>>>>>>>>>>> >>>>>>>>>>>> We intercept #BP while in guest debugging mode. As VM exists due to >>>>>>>>>>>> intercepted exceptions do not necessarily come with valid >>>>>>>>>>>> idt_vectoring, we have to update event_exit_inst_len explicitly in such >>>>>>>>>>>> cases. At least in the absence of migration, this ensures that >>>>>>>>>>>> re-injections of #BP will find and use the correct instruction length. >>>>>>>>>>>> >>>>>>>>>>> event_exit_inst_len is only used for event reinjection. Since event >>>>>>>>>>> intercepted here will not be reinjected why updating event_exit_inst_len >>>>>>>>>>> is needed here? >>>>>>>>>> In guest debugging mode a #BP exception is always reported to user space >>>>>>>>>> to find out what caused it. If it was the guest itself, the exception is >>>>>>>>>> reinjected, on older kernels via KVM_SET_GUEST_DEBUG and since 2.6.33 >>>>>>>>>> via KVM_SET_VCPU_EVENTS (the latter requires some qemu patch that I will >>>>>>>>>> post later). >>>>>>>>>> >>>>>>>>>> As we currently do not update event_exit_inst_len on #BP exits, >>>>>>>>>> reinjecting fails unless event_exit_inst_len happens to be 1 from some >>>>>>>>>> other exit. >>>>>>>>>> >>>>>>>>> Hmm, how does it work on SVM then where we do not have >>>>>>>>> event_exit_inst_len so execution will resume on the same rip that caused >>>>>>>>> #BP after event reinjection? >>>>>>>>> >>>>>>>> Maybe not at all. I don't think I've tested this scenario on amd so far. >>>>>>>> Guess it needs some special handling in svm to move rip after the int3 >>>>>>>> when requesting to inject #BP. >>>>>>>> >>>>>>> This will work for VMX too, no? So may be we should design something >>>>>>> that will work for both VMX and SVM before applying patches that make >>>>>>> oly VMX work? >>>>>> VMX used to work, so my patch is actually a regression fix. I bet this >>>>>> was accidentally broken while cleaning up the interrupt handling of VMX. >>>>>> >>>>> VMX used to always reexecute instruction. >>>> ...since 66fd3f7f90. And that was what broke this guest debugging corner >>>> case. >>>> >>> I see. And I see why it worked, but it shouldn't have been working for >>> SVM. I prefer to look for general solution here that works for SVM/VMX. >> I don't see the need to emulate INT3 for the sake of unification. VMX >> works today (with this patch), and SVM might work without further >> efforts, at least on modern hosts: >> >> "Software interrupts cannot be properly injected if the processor does >> not support the NextRIP field, indicated by EDX[3] = 1 as returned by >> CPUID function 8000_000A. Hypervisor software should emulate the event >> injection of software interrupts if NextRIP is not supported." >> >> (right below the paragraph I cited before) >> >> I assume, INT3 can be considered as software interrupt as well in this >> context. >> > Lets check if SVM works. I can do that if you tell me how. - Fire up some Linux guest with gdb installed - Attach gdb to gdbstub of the VM - Set a soft breakpoint in guest kernel, ideally where it does not immediately trigger, e.g. on sys_reboot (use grep sys_reboot /proc/kallsyms if you don't have symbols for the guest kernel) - Start gdb /bin/true in the guest - run As gdb sets some automatic breakpoints, this already exercises the reinjection of #BP. > My concern is > that if SVM doesn't work we will write another way to handle this > situation that will work on SVM and VMX and then we will have code in > VMX that is not needed, but we have to support it. I understand your concerns, but I do not share them ATM. I bet we will need some workaround for older SVM, but it will be some SVM-only thing. Anyway, we can wait with this patch a few days until we know more. Jan
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