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

[Heinrich Schuchardt]

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@xxxxxx>
> > ---
> >   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