On Sat, 2012-12-15 at 10:07 +0200, Gleb Natapov wrote:
> On Fri, Dec 14, 2012 at 04:42:36PM -0700, Alex Williamson wrote:
> >
> > I don't really know what I'm doing messing around with realmode
> > exception handling, but are we missing something like this:
> >
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -4332,7 +4332,8 @@ static int handle_exception(struct kvm_vcpu *vcpu)
> > if (vcpu->arch.halt_request) {
> > vcpu->arch.halt_request = 0;
> > return kvm_emulate_halt(vcpu);
> > - }
> > + } else if (vcpu->mmio_needed)
> > + return 0;
> > return 1;
> > }
> >
> >
> > The background is that I was playing around with when the MMIO regions
> > of PCI assigned devices get direct mapped through KVM and start using
> > memory slots and I found that the boot ROM on an e1000e card that I
> > often use for testing causes the VM to hang when not directly mapped
> > through a memory slot. This happens with both pci-assign and vfio-pci.
> > Debugging further, I see the kernel instruction emulator decodes this
> > and sets all the necessary flags and fragments for userspace to complete
> > the access, but since it happened in real mode, vcpu_enter_guest always
> > returns 1 and therefore KVM_RUN never exits to userspace and KVM gets
> > stuck in an infinite loop re-handling this instruction.
> >
> > The above is pretty trivial and allows the ROM to work again. Is it
> > correct? What other exceptions are we missing here that should allow an
> > exit? This particular access generated a GP_VECTOR exception if we want
> > to key on that somewhere. Thanks,
> >
> As far as I see the patch is nop. If handle_rmode_exception() returns 1
> mmio_needed should be 0. Also if handle_rmode_exception() makes IO
> according to the code qemu should exit with "KVM: exception 13 exit
> (error code 0x0)" or something. Can you provide a ftrace for the hang?
The real mode vector is GP_VECTOR, so handle_rmode_exception returns 1
even though emulate_instruction returns EMULATE_DO_MMIO. Maybe the
point at which handle_rmode_exception gets EMULATE_DO_MMIO from
emulate_instruction is the point at which we should return 0. I'll run
an ftrace if it's still not clear how we're getting here. Thanks,
Alex
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