Re: [PATCH kvmtool v2 1/2] cpu: vmexit: Handle KVM_EXIT_UNKNOWN exit reason correctly

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

Will Deacon <will@kernel.org> writes:

> On Mon, Feb 24, 2025 at 02:39:59PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> The return value for kernel VM exit handlers is confusing and has led to
>> errors in different kernel exit handlers. A return value of 0 indicates
>> a return to the VMM, whereas a return value of 1 indicates resuming
>> execution in the guest. Some handlers mistakenly return 0 to force a
>> return to the guest.
>> 
>> This worked in kvmtool because the exit_reason defaulted to
>> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit
>> reason. However, forcing a VMM exit with error on KVM_EXIT_UNKNOWN
>> exit_reson would help catch these bugs early.
>> 
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>  kvm-cpu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kvm-cpu.c b/kvm-cpu.c
>> index f66dcd07220c..7c62bfc56679 100644
>> --- a/kvm-cpu.c
>> +++ b/kvm-cpu.c
>> @@ -170,7 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>>  
>>  		switch (cpu->kvm_run->exit_reason) {
>>  		case KVM_EXIT_UNKNOWN:
>> -			break;
>> +			goto panic_kvm;
>>  		case KVM_EXIT_DEBUG:
>>  			kvm_cpu__show_registers(cpu);
>>  			kvm_cpu__show_code(cpu);
>> -- 
>> 2.43.0
>
> This breaks SMP boot on my x86 machine:
>
> # ./lkvm run
> ...
> [    0.628472] smp: Bringing up secondary CPUs ...
> [    0.630401] smpboot: x86: Booting SMP configuration:
>   Error: KVM exit reason: 0 ("KVM_EXIT_UNKNOWN")
>   Error: KVM exit code: 0
>

I can recreate this with distro kernel as the guest kernel but not with
CONFIG_SMP enabled with kvm_guest.config. I will have to find out what
is causing that KVM_EXIT_UNKNOWN on the secondary cpu boot.

-aneesh