Messages in this thread | | | Subject | Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded | From | Alexander Graf <> | Date | Fri, 6 Sep 2019 15:16:07 +0200 |
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On 06.09.19 15:12, Christoffer Dall wrote: > On Fri, Sep 06, 2019 at 02:08:15PM +0200, Alexander Graf wrote: >> >> >> On 06.09.19 10:00, Christoffer Dall wrote: >>> On Thu, Sep 05, 2019 at 02:09:18PM +0100, Marc Zyngier wrote: >>>> On 05/09/2019 10:22, Christoffer Dall wrote: >>>>> On Thu, Sep 05, 2019 at 09:56:44AM +0100, Peter Maydell wrote: >>>>>> On Thu, 5 Sep 2019 at 09:52, Marc Zyngier <maz@kernel.org> wrote: >>>>>>> >>>>>>> On Thu, 05 Sep 2019 09:16:54 +0100, >>>>>>> Peter Maydell <peter.maydell@linaro.org> wrote: >>>>>>>> This is true, but the problem is that barfing out to userspace >>>>>>>> makes it harder to debug the guest because it means that >>>>>>>> the VM is immediately destroyed, whereas AIUI if we >>>>>>>> inject some kind of exception then (assuming you're set up >>>>>>>> to do kernel-debug via gdbstub) you can actually examine >>>>>>>> the offending guest code with a debugger because at least >>>>>>>> your VM is still around to inspect... >>>>>>> >>>>>>> To Christoffer's point, I find the benefit a bit dubious. Yes, you get >>>>>>> an exception, but the instruction that caused it may be completely >>>>>>> legal (store with post-increment, for example), leading to an even >>>>>>> more puzzled developer (that exception should never have been >>>>>>> delivered the first place). >>>>>> >>>>>> Right, but the combination of "host kernel prints a message >>>>>> about an unsupported load/store insn" and "within-guest debug >>>>>> dump/stack trace/etc" is much more useful than just having >>>>>> "host kernel prints message" and "QEMU exits"; and it requires >>>>>> about 3 lines of code change... >>>>>> >>>>>>> I'm far more in favour of dumping the state of the access in the run >>>>>>> structure (much like we do for a MMIO access) and let userspace do >>>>>>> something about it (such as dumping information on the console or >>>>>>> breaking). It could even inject an exception *if* the user has asked >>>>>>> for it. >>>>>> >>>>>> ...whereas this requires agreement on a kernel-userspace API, >>>>>> larger changes in the kernel, somebody to implement the userspace >>>>>> side of things, and the user to update both the kernel and QEMU. >>>>>> It's hard for me to see that the benefit here over the 3-line >>>>>> approach really outweighs the extra effort needed. In practice >>>>>> saying "we should do this" is saying "we're going to do nothing", >>>>>> based on the historical record. >>>>>> >>>>> >>>>> How about something like the following (completely untested, liable for >>>>> ABI discussions etc. etc., but for illustration purposes). >>>>> >>>>> I think it raises the question (and likely many other) of whether we can >>>>> break the existing 'ABI' and change behavior for missing ISV >>>>> retrospectively for legacy user space when the issue has occurred? >>>>> Someone might have written code that reacts to the -ENOSYS, so I've >>>>> taken the conservative approach for this for the time being. >>>>> >>>>> >>>>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h >>>>> index 8a37c8e89777..19a92c49039c 100644 >>>>> --- a/arch/arm/include/asm/kvm_host.h >>>>> +++ b/arch/arm/include/asm/kvm_host.h >>>>> @@ -76,6 +76,14 @@ struct kvm_arch { >>>>> /* Mandated version of PSCI */ >>>>> u32 psci_version; >>>>> + >>>>> + /* >>>>> + * If we encounter a data abort without valid instruction syndrome >>>>> + * information, report this to user space. User space can (and >>>>> + * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is >>>>> + * supported. >>>>> + */ >>>>> + bool return_nisv_io_abort_to_user; >>>>> }; >>>>> #define KVM_NR_MEM_OBJS 40 >>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >>>>> index f656169db8c3..019bc560edc1 100644 >>>>> --- a/arch/arm64/include/asm/kvm_host.h >>>>> +++ b/arch/arm64/include/asm/kvm_host.h >>>>> @@ -83,6 +83,14 @@ struct kvm_arch { >>>>> /* Mandated version of PSCI */ >>>>> u32 psci_version; >>>>> + >>>>> + /* >>>>> + * If we encounter a data abort without valid instruction syndrome >>>>> + * information, report this to user space. User space can (and >>>>> + * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is >>>>> + * supported. >>>>> + */ >>>>> + bool return_nisv_io_abort_to_user; >>>>> }; >>>>> #define KVM_NR_MEM_OBJS 40 >>>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >>>>> index 5e3f12d5359e..a4dd004d0db9 100644 >>>>> --- a/include/uapi/linux/kvm.h >>>>> +++ b/include/uapi/linux/kvm.h >>>>> @@ -235,6 +235,7 @@ struct kvm_hyperv_exit { >>>>> #define KVM_EXIT_S390_STSI 25 >>>>> #define KVM_EXIT_IOAPIC_EOI 26 >>>>> #define KVM_EXIT_HYPERV 27 >>>>> +#define KVM_EXIT_ARM_NISV 28 >>>>> /* For KVM_EXIT_INTERNAL_ERROR */ >>>>> /* Emulate instruction failed. */ >>>>> @@ -996,6 +997,7 @@ struct kvm_ppc_resize_hpt { >>>>> #define KVM_CAP_ARM_PTRAUTH_ADDRESS 171 >>>>> #define KVM_CAP_ARM_PTRAUTH_GENERIC 172 >>>>> #define KVM_CAP_PMU_EVENT_FILTER 173 >>>>> +#define KVM_CAP_ARM_NISV_TO_USER 174 >>>>> #ifdef KVM_CAP_IRQ_ROUTING >>>>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c >>>>> index 35a069815baf..2ce94bd9d4a9 100644 >>>>> --- a/virt/kvm/arm/arm.c >>>>> +++ b/virt/kvm/arm/arm.c >>>>> @@ -98,6 +98,26 @@ int kvm_arch_check_processor_compat(void) >>>>> return 0; >>>>> } >>>>> +int kvm_vm_ioctl_enable_cap(struct kvm *kvm, >>>>> + struct kvm_enable_cap *cap) >>>>> +{ >>>>> + int r; >>>>> + >>>>> + if (cap->flags) >>>>> + return -EINVAL; >>>>> + >>>>> + switch (cap->cap) { >>>>> + case KVM_CAP_ARM_NISV_TO_USER: >>>>> + r = 0; >>>>> + kvm->arch.return_nisv_io_abort_to_user = true; >>>>> + break; >>>>> + default: >>>>> + r = -EINVAL; >>>>> + break; >>>>> + } >>>>> + >>>>> + return r; >>>>> +} >>>>> /** >>>>> * kvm_arch_init_vm - initializes a VM data structure >>>>> @@ -196,6 +216,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) >>>>> case KVM_CAP_MP_STATE: >>>>> case KVM_CAP_IMMEDIATE_EXIT: >>>>> case KVM_CAP_VCPU_EVENTS: >>>>> + case KVM_CAP_ARM_NISV_TO_USER: >>>>> r = 1; >>>>> break; >>>>> case KVM_CAP_ARM_SET_DEVICE_ADDR: >>>>> @@ -673,6 +694,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run) >>>>> ret = kvm_handle_mmio_return(vcpu, vcpu->run); >>>>> if (ret) >>>>> return ret; >>>>> + } else if (run->exit_reason == KVM_EXIT_ARM_NISV) { >>>>> + kvm_inject_undefined(vcpu); >>>> >>>> Just to make sure I understand: Is the expectation here that userspace >>>> could clear the exit reason if it managed to handle the exit? And >>>> otherwise we'd inject an UNDEF on reentry? >>>> >>> >>> Yes, but I think we should change that to an external abort. I'll test >>> something and send a proper patch with more clear documentation. >> >> Why not leave the injection to user space in any case? API wise there is no >> need to be backwards compatible, as we require the CAP to be enabled, right? >> > > I'd prefer leaving it to userspace to worry about, but I thought Peter > said that had been problematic historically, which I took at face value, > but I could have misunderstood. > > If QEMU, kvmtool, and whatever the crazy^H cool kids are using in > userspace these days are happy emulating the exception, then that's a > viable approach. The main concern I have with that is whether they'll > all get it right, and since we already have the code in the kernel to do > this, it might make sense to re-use the kernel logic for it.
You could make the same argument about injecting an #SError on an out of bounds access to MMIO.
If injecting a fault is too complicated, we should fix that rather than create an unbalanced user space interface :).
> I'll leave it in for v1 of the patch, and if based on how that code and > interface looks like, we agree it's better to leave it to userspace, I > can remove it in v2.
Sure, works for me :). Please CC me on v1 so I can comment on it ;)
Alex
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