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

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Date:   Thu, 5 Sep 2019 14:01:36 +0200
From:   Heinrich Schuchardt <xypron.glpk@....de>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Daniel P. Berrangé <berrange@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be
 decoded

On 9/5/19 11:20 AM, Stefan Hajnoczi wrote:
> On Wed, Sep 04, 2019 at 08:07:36PM +0200, Heinrich Schuchardt wrote:
>> If an application tries to access memory that is not mapped, an error
>> ENOSYS, "load/store instruction decoding not implemented" may occur.
>> QEMU will hang with a register dump.
>>
>> Instead create a data abort that can be handled gracefully by the
>> application running in the virtual environment.
>>
>> Now the virtual machine can react to the event in the most appropriate
>> way - by recovering, by writing an informative log, or by rebooting.
>>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
>> ---
>>   virt/kvm/arm/mmio.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
>> index a8a6a0c883f1..0cbed7d6a0f4 100644
>> --- a/virt/kvm/arm/mmio.c
>> +++ b/virt/kvm/arm/mmio.c
>> @@ -161,8 +161,8 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>   		if (ret)
>>   			return ret;
>>   	} else {
>> -		kvm_err("load/store instruction decoding not implemented\n");
>> -		return -ENOSYS;
>> +		kvm_inject_dabt(vcpu, kvm_vcpu_get_hfar(vcpu));
>> +		return 1;
>
> I see this more as a temporary debugging hack than something to merge.
>
> It sounds like in your case the guest environment provided good
> debugging information and you preferred it over debugging this from the
> host side.  That's fine, but allowing the guest to continue running in
> the general case makes it much harder to track down the root cause of a
> problem because many guest CPU instructions may be executed after the
> original problem occurs.  Other guest software may fail silently in
> weird ways.  IMO it's best to fail early.
>
> Stefan
>

As virtual machine are ubiquitous, expect also mission critical system
to run on them. At development time halting a machine may be a good
idea. In production this is often the worst solution. Rebooting may be
essential for survival.

For an anecdotal example see:
https://www.hq.nasa.gov/alsj/a11/a11.1201-pa.html

I am convinced that leaving it to the guest to decide how to react is
the best choice.

Best regards

Heinrich