Re: [PATCH][next] KVM: x86: remove redundant WARN_ON check of an unsigned less than zero

[Paolo Bonzini]

On 08/02/20 01:47, Sean Christopherson wrote:
> On Fri, Feb 07, 2020 at 11:18:13PM +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>>
>> The check cpu->hv_clock.system_time < 0 is redundant since system_time
>> is a u64 and hence can never be less than zero. Remove it.
>>
>> Addresses-Coverity: ("Macro compares unsigned to 0")
>> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>> ---
>>  arch/x86/kvm/x86.c | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index fbabb2f06273..d4967ac47e68 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2448,7 +2448,6 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
>>  	vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
>>  	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
>>  	vcpu->last_guest_tsc = tsc_timestamp;
>> -	WARN_ON(vcpu->hv_clock.system_time < 0);
> 
> Don't know this code well, but @kernel_ns and @v->kvm->arch.kvmclock_offset
> are both s64, so maybe this was intended and/or desirable?
> 
> 	WARN_ON((s64)vcpu->hv_clock.system_time < 0);

Yes, that's related to the bugfix where kvmclock would get negative.  I
queued the patch with the (s64) cast added.  Thanks to both of you!

Paolo