Marcelo Tosatti wrote: > On Thu, Feb 04, 2010 at 04:41:44PM +0100, Jan Kiszka wrote: >> Jan Kiszka wrote: >>> Marcelo Tosatti wrote: >>>> On Thu, Feb 04, 2010 at 01:33:50AM +0100, Jan Kiszka wrote: >>>>> Marcelo Tosatti wrote: >>>>>> On Wed, Feb 03, 2010 at 10:29:45PM +0100, Jan Kiszka wrote: >>>>>>> So far we synchronized any dirty VCPU state back into the kernel before >>>>>>> updating the guest debug state. This was a tribute to a deficit in x86 >>>>>>> kernels before 2.6.33. But as this is an arch-dependent issue, it is >>>>>>> better handle in the x86 part of KVM and remove the writeback point for >>>>>>> generic code. >>>>>> Jan, >>>>>> >>>>>> This patch breaks migration. >>>>> Can you elaborate what you did? I can't reproduce, and I do not see any >>>>> conceptual issue (given that guest debugging conflicts with migration >>>>> anyway). >>>> kvm-autotest fails (migration only, install is ok, both Linux and Win >>>> guests). Not sure why, perhaps the unconditional KVM_SET_GUEST_DEBUG >>>> corrupts state somehow? >>>> >>>> Tested with io thread enabled. >>> That's this default-off thing, so... OK, confirmed, investigating. >>> >> Heisenbug: It first also popped up (in form of a frozen migration >> target) after removing this patch, but now it's totally unreproducible, >> whatever patch I apply or revert from my series. Base is current master. >> >> I tend to think there is a hidden issue of iothread vs. migration, >> unrelated to this patch. > > Probably many :) > > Do you have c5f32c99c6855d466737daf1cd262e7e92062f87 (from qemu-kvm.git > uq/master) in? Yes. And that might have been the reason why some early tests failed when it was no yet applied here. > > With kvm-autotest the failure is not sporadic (and the above commit > applied): with KVM_SET_GUEST_DEBUG in arch_put_regs all migration > tests fail, without, all of them succeed. > > So env->kvm_guest_debug has been zeroed by cpu_x86_init, which means > the writeback via KVM_SET_GUEST_DEBUG does almost nothing. It does > get_rflags and set_rflags in the kernel. Hmm, it also copies debug regs around... BTW, where do we save/restore dr0..7 between kernel and user space? But that should not be a problem, both shadow as well as effective regs should be properly initialized, specifically for a newly created VCPU. > > Test box is off, but the synchronous writeback via qemu_system_reset > in main, after machine and vcpu thread initialization, might be > problematic. But it would be nice to understand this. > > Unrelated to this problem, won't put_vcpu_events, which is executed > after KVM_SET_GUEST_DEBUG, overwrite any queued debug exceptions? Good point, SET_GUEST_DEBUG should be last in the writeback for that reason. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- 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
