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

[Heinrich Schuchardt]

On 9/5/19 10:03 AM, Marc Zyngier wrote:
> [Please use my kernel.org address. My arm.com address will disappear shortly]
>
> On Wed, 04 Sep 2019 19:07:36 +0100,
> Heinrich Schuchardt <xypron.glpk@xxxxxx> 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;
>
> How can you tell that the access would fault? You have no idea at that
> stage (the kernel doesn't know about the MMIO ranges that userspace
> handles). All you know is that you're faced with a memory access that
> you cannot emulate in the kernel. Injecting a data abort at that stage
> is not something that the architecture allows.
>
> If you want to address this, consider forwarding the access to
> userspace. You'll only need an instruction decoder (supporting T1, T2,
> A32 and A64) and a S1 page table walker (one per page table format,
> all three of them) to emulate the access (having taken care of
> stopping all the other vcpus to make sure there is no concurrent
> modification of the page tables). You'll then be able to return the
> result of the access back to the kernel.

If I understand you right, the problem has to be fixed in QEMU and not
in KVM.

Is there an example of such a forwarding already available in QEMU?

> Of course, the best thing would be to actually fix the guest so that
> it doesn't use non-emulatable MMIO accesses. In general, that the sign
> of a bug in low-level accessors.

My problem was to find out where in my guest (U-Boot running UEFI SCT)
the problem occurred. With this patch U-Boot gave me the relative
address in Shell.efi and I was able to locate what was wrong in U-Boot's
UEFI implementation.

Thanks for reviewing.

Heinrich