linux-kernel - Re: [PATCH][next] KVM: x86: remove redundant WARN

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Message-ID: <881ac78f-4afa-49c1-7684-85b196656051@canonical.com>
Date:   Sat, 8 Feb 2020 12:09:12 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] KVM: x86: remove redundant WARN_ON check of an
 unsigned less than zero

On 08/02/2020 00:47, Sean Christopherson wrote:
> On Fri, Feb 07, 2020 at 11:18:13PM +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> 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@...onical.com>
>> ---
>>  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?

Something is definitely not correct for sure.

> 
> 	WARN_ON((s64)vcpu->hv_clock.system_time < 0);
> 	
> 
>>  	/* If the host uses TSC clocksource, then it is stable */
>>  	pvclock_flags = 0;
>> -- 
>> 2.24.0
>>