Re: [PATCH kvmtool 2/2] cpu: vmexit: Handle KVM_EXIT_MEMORY_FAULT correctly

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Alexandru Elisei <alexandru.elisei@arm.com> writes:

> Hi,
>
> On Tue, Feb 11, 2025 at 01:08:52PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> Linux kernel documentation states:
>>
>> "Note! KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in
>> that it accompanies a return code of '-1', not '0'! errno will always be
>> set to EFAULT or EHWPOISON when KVM exits with KVM_EXIT_MEMORY_FAULT,
>> userspace should assume kvm_run.exit_reason is stale/undefined for all
>> other error numbers." "
>>
>> Update the KVM_RUN ioctl error handling to correctly handle
>> KVM_EXIT_MEMORY_FAULT.
>
> I've tried to follow how kvmtool handles KVM_EXIT_MEMORY_FAULT before and after
> this patch.
>
> Before: calls die_perror().
> After: prints more information about the error, in kvm_cpu_thread().
>
> Is that what you want? Because "correctly handle KVM_EXIT_MEMORY_FAULT" can be
> interpreted as kvmtool resolving the memory fault, which is something that
> kvmtool does not do.
>

That is correct. The changes to enable the handling of
KVM_EXIT_MEMORY_FAULT is not yet part of upstream [1]. But then the
return value for KVM_RUN ioctl() is defined as part of
Documentation/virt/kvm/api.rst in the Linux kernel.

[1] https://gitlab.arm.com/linux-arm/kvmtool-cca/-/blob/cca/v4/arm/kvm-cpu.c?ref_type=heads#L247

>
> Also, can you update kvm_exit_reasons with KVM_EXIT_MEMORY_FAULT, because
> otherwise kvm_cpu_thread() will segfault when it tries to access
> kvm_exit_reasons[KVM_EXIT_MEMORY_FAULT].
>

Will update.

-aneesh