From: Marc Zyngier <maz@kernel.org>
To: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: "James Morse" <james.morse@arm.com>,
"Julien Thierry" <julien.thierry@arm.com>,
"Suzuki K Pouloze" <suzuki.poulose@arm.com>,
"Peter Maydell" <peter.maydell@linaro.org>,
"Stefan Hajnoczi" <stefanha@redhat.com>,
"\"Daniel P . Berrangé\"" <berrange@redhat.com>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded
Date: Thu, 05 Sep 2019 10:11:38 +0100 [thread overview]
Message-ID: <86lfv3rtth.wl-maz@kernel.org> (raw)
In-Reply-To: <754fb77a-aace-e0aa-a5bb-a6c6bcff9890@gmx.de>
On Thu, 05 Sep 2019 09:28:42 +0100,
Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
>
> 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@gmx.de> 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@gmx.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;
> >
> > 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.
It is a shared responsibility.
> Is there an example of such a forwarding already available in QEMU?
Yes. That's how we ask userspace (QEMU) to perform a MMIO access on
behalf of the guest (see how the run structure is populated and the
vcpu thread returns to userspace).
> >
> > 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.
I understand that there is a need for more precise information. I'm
just wary of adding debug features that become something that people
rely on, despite being in contradiction with the architecture.
I can help with the kernel side of the reporting, should someone want
to tackle the userspace side of it.
Thanks,
M.
--
Jazz is not dead, it just smells funny.
next prev parent reply other threads:[~2019-09-05 9:11 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-04 18:07 [PATCH 1/1] KVM: inject data abort if instruction cannot be decoded Heinrich Schuchardt
2019-09-05 8:03 ` Marc Zyngier
2019-09-05 8:16 ` Peter Maydell
2019-09-05 8:25 ` Christoffer Dall
2019-09-05 8:32 ` Peter Maydell
2019-09-05 8:48 ` Heinrich Schuchardt
2019-09-05 8:52 ` Marc Zyngier
2019-09-05 8:56 ` Peter Maydell
2019-09-05 9:15 ` Marc Zyngier
2019-09-05 9:22 ` Christoffer Dall
2019-09-05 13:09 ` Marc Zyngier
2019-09-06 8:00 ` Christoffer Dall
2019-09-06 12:08 ` Alexander Graf
2019-09-06 12:34 ` Marc Zyngier
2019-09-06 13:02 ` [UNVERIFIED SENDER] " Alexander Graf
2019-09-06 13:12 ` Christoffer Dall
2019-09-06 13:16 ` Alexander Graf
2019-09-06 13:31 ` Peter Maydell
2019-09-06 13:41 ` Alexander Graf
2019-09-06 13:50 ` Peter Maydell
2019-09-06 14:12 ` Alexander Graf
2019-09-06 13:44 ` Christoffer Dall
2019-09-05 13:25 ` Heinrich Schuchardt
2019-09-06 7:58 ` Christoffer Dall
2019-09-05 8:28 ` Heinrich Schuchardt
2019-09-05 9:11 ` Marc Zyngier [this message]
2019-09-05 9:20 ` Stefan Hajnoczi
2019-09-05 9:23 ` Daniel P. Berrangé
2019-09-05 12:01 ` Heinrich Schuchardt
2019-09-05 12:16 ` Christoffer Dall
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