Re: [PATCH kvmtool 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 Tue, Feb 11, 2025 at 01:08:51PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> The return value for the KVM_RUN ioctl 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.
>
> Oops. Did any of those broken handlers reach mainline?
>

Not that I noticed. We do have patches in review.

https://lore.kernel.org/all/20241212155610.76522-18-steven.price@arm.com


>> 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 KVM panic on an unknown exit reason would
>> help catch these bugs early.
>> 
>> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
>> ---
>>  kvm-cpu.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/kvm-cpu.c b/kvm-cpu.c
>> index f66dcd07220c..66e30ba54e26 100644
>> --- a/kvm-cpu.c
>> +++ b/kvm-cpu.c
>> @@ -170,6 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>>  
>>  		switch (cpu->kvm_run->exit_reason) {
>>  		case KVM_EXIT_UNKNOWN:
>> +			goto panic_kvm;
>>  			break;
>
> The break is no longer needed.
>

ok.

-aneesh