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

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Date:   Fri, 6 Sep 2019 13:34:48 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Alexander Graf <graf@...zon.com>,
        Christoffer Dall <christoffer.dall@....com>
Cc:     Daniel P. Berrangé <berrange@...hat.com>,
        Heinrich Schuchardt <xypron.glpk@....de>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        kvmarm@...ts.cs.columbia.edu,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be
 decoded

On 06/09/2019 13:08, 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:

[...]

>>>> @@ -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?
> 
> IMHO it should be 100% a policy decision in user space whether to 
> emulate and what type of exception to inject, if anything.

The exception has to be something that the trapped instruction can
actually generate. An UNDEF is definitely wrong, as the guest would have
otherwise UNDEF'd at EL1, and KVM would have never seen it. You cannot
deviate from the rule of architecture, and userspace feels like the
wrong place to enforce it.

	M.
-- 
Jazz is not dead, it just smells funny...