Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded

From: Alexander Graf <[email-masked]>
Date: Fri, 6 Sep 2019 15:16:07 +0200

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 <[email-masked]> wrote:
>>>>>>>
>>>>>>> On Thu, 05 Sep 2019 09:16:54 +0100,
>>>>>>> Peter Maydell <[email-masked]> 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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