Re: [PATCH rfc] hvf: arm: Inject SEA when executing insn in invalid memory range

Zenghui Yu
2026-03-16T10:55:20Z

Hi Alex,

On 3/16/26 5:40 PM, Alex Bennée wrote:
> Zenghui Yu <zenghui.yu@linux.dev> writes:
>
> > It seems that hvf doesn't deal with the abort generated when guest tries to
> > execute instructions outside of the valid physical memory range, for
> > unknown reason. The abort is forwarded to userspace and QEMU doesn't handle
> > it either, which ends up with faulting on the same instruction infinitely.
> >
> > This was noticed by the kvm-unit-tests/selftest-vectors-kernel failure:
> >
> > timeout -k 1s --foreground 90s /opt/homebrew/bin/qemu-system-aarch64 \
> > -nodefaults -machine virt -accel hvf -cpu host \
> > -device virtio-serial-device -device virtconsole,chardev=ctd \
> > -chardev testdev,id=ctd -device pci-testdev -display none \
> > -serial stdio -kernel arm/selftest.flat -smp 1 -append vectors-kernel
>
> Have you got patches for teaching kvm-unit-tests about hvf or are you
> running these all manually? I tried building on the Mac we have but it
> failed the build and the docs only mention x86.

No more patches. This is how I build and test it on my M1:

./configure --arch=aarch64 --page-size=16k --cross-prefix=aarch64-elf-
make && make standalone
QEMU=qemu-system-aarch64 ACCEL=hvf ./run_tests.sh

The command line above is copied from logs/selftest-vectors-kernel.log.

> > PASS: selftest: vectors-kernel: und
> > PASS: selftest: vectors-kernel: svc
> > qemu-system-aarch64: 0xffffc000: unhandled exception ec=0x20
> > qemu-system-aarch64: 0xffffc000: unhandled exception ec=0x20
> > qemu-system-aarch64: 0xffffc000: unhandled exception ec=0x20
>
> I think this is running:
>
> static bool check_pabt(void)
> {
> enum vector v = check_vector_prep();
>
> install_exception_handler(v, ESR_EL1_EC_IABT_EL1, pabt_handler);
>
> test_exception("adrp x9, check_pabt_invalid_paddr\n"
> "add x9, x9, :lo12:check_pabt_invalid_paddr\n"
> "ldr x9, [x9]\n",
> "blr x9\n",
> "", "x9", "x30");
>
> install_exception_handler(v, ESR_EL1_EC_IABT_EL1, NULL);
>
> return pabt_works;
> }
>
> which is expecting 0x21 - instruction abort at the same exception level.
> I wonder why there is the difference.

It's the guest that executes an instruction outside of the valid memory
range. From hypervisor's perspective, it's an instruction abort from a
lower EL, so we get the syndrome saying that EC=0x20. We then decide
that we should emulate the hardware behavior by injecting an SEA into
the insane guest.

And from guest's perspective, it's an instruction abort caused by itself
(thus EC=0x21 - instruction abort taken without a change in EL).

> > [...]
> >
> > It's apparent that the guest is braindead and it's unsure what prevents hvf
> > from injecting an abort directly in that case. Try to deal with the insane
> > guest in QEMU by injecting an SEA back into it in the EC_INSNABORT
> > emulation path.
> >
> > Signed-off-by: Zenghui Yu <zenghui.yu@linux.dev>
> > ---
> > target/arm/hvf/hvf.c | 23 +++++++++++++++++++++++
> > 1 file changed, 23 insertions(+)
> >
> > diff --git a/target/arm/hvf/hvf.c b/target/arm/hvf/hvf.c
> > index aabc7d32c1..54d6ea469c 100644
> > --- a/target/arm/hvf/hvf.c
> > +++ b/target/arm/hvf/hvf.c
> > @@ -2332,9 +2332,32 @@ static int hvf_handle_exception(CPUState *cpu, hv_vcpu_exit_exception_t *excp)
> > bool ea = (syndrome >> 9) & 1;
> > bool s1ptw = (syndrome >> 7) & 1;
> > uint32_t ifsc = (syndrome >> 0) & 0x3f;
> > + uint64_t ipa = excp->physical_address;
> > + AddressSpace *as = cpu_get_address_space(cpu, ARMASIdx_NS);
> > + hwaddr xlat;
> > + MemoryRegion *mr;
> > +
> > + cpu_synchronize_state(cpu);
> >
> > trace_hvf_insn_abort(env->pc, set, fnv, ea, s1ptw, ifsc);
> >
> > + /*
> > + * TODO: If s1ptw, this is an error in the guest os page tables.
> > + * Inject the exception into the guest.
> > + */
> > + assert(!s1ptw);
> > +
> > + mr = address_space_translate(as, ipa, &xlat, NULL, false,
> > + MEMTXATTRS_UNSPECIFIED);
> > + if (unlikely(!memory_region_is_ram(mr))) {
> > + uint32_t syn;
> > +
> > + /* inject an SEA back into the guest */
> > + syn = syn_insn_abort(arm_current_el(env) == 1, ea, false, 0x10);
> > + hvf_raise_exception(cpu, EXCP_PREFETCH_ABORT, syn, 1);
> > + break;
> > + }
> > +
> > /* fall through */
> > }
> > default:
>
> I need the check the exception paths for KVM and TCG. I guess for the
> KVM case it is all in the kernel?

Yup, in kvm_handle_guest_abort().

Thanks,
Zenghui