linux-kernel - Re: [PATCH v9 05/14] KVM: X86: Implement ring-based dirty memory tracking

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Message-ID: <20200525152649.GF1058657@xz-x1>
Date:   Mon, 25 May 2020 11:26:49 -0400
From:   Peter Xu <peterx@...hat.com>
To:     kbuild test robot <lkp@...el.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kbuild-all@...ts.01.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Christophe de Dinechin <dinechin@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>
Subject: Re: [PATCH v9 05/14] KVM: X86: Implement ring-based dirty memory
 tracking

On Mon, May 25, 2020 at 10:48:08AM +0800, kbuild test robot wrote:
> Hi Peter,
> 
> Thank you for the patch! Perhaps something to improve:
> 
> [auto build test WARNING on vhost/linux-next]
> [also build test WARNING on linus/master v5.7-rc7]
> [cannot apply to kvm/linux-next tip/auto-latest linux/master next-20200522]
> [if your patch is applied to the wrong git tree, please drop us a note to help
> improve the system. BTW, we also suggest to use '--base' option to specify the
> base tree in git format-patch, please see https://stackoverflow.com/a/37406982]
> 
> url:    https://github.com/0day-ci/linux/commits/Peter-Xu/KVM-Dirty-ring-interface/20200524-070926
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git linux-next
> config: s390-randconfig-s002-20200524 (attached as .config)
> compiler: s390-linux-gcc (GCC) 9.3.0
> reproduce:
>         # apt-get install sparse
>         # sparse version: v0.6.1-240-gf0fe1cd9-dirty
>         # save the attached .config to linux build tree
>         make W=1 C=1 ARCH=s390 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'
> 
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kbuild test robot <lkp@...el.com>
> 
> All warnings (new ones prefixed by >>, old ones prefixed by <<):
> 
> arch/s390/kvm/../../../virt/kvm/kvm_main.c: In function 'kvm_page_in_dirty_ring':
> >> arch/s390/kvm/../../../virt/kvm/kvm_main.c:2932:16: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
> 2932 |  return (pgoff >= KVM_DIRTY_LOG_PAGE_OFFSET) &&
> |                ^~

Should be a false positive, since when KVM_DIRTY_LOG_PAGE_OFFSET==0 (true for
s390) then the code won't reach here at all due to the previous check [1].

I thought gcc should be able to directly remove the below code when it sees "if
(1) return false;", but it seems not...

I wanted to avoid using "#if" macros because I think it always makes the code
harder to read (especially when nested).  Maybe it's still ok to use one more
time here.

Thanks,

> 
> vim +2932 arch/s390/kvm/../../../virt/kvm/kvm_main.c
> 
>   2926	
>   2927	static bool kvm_page_in_dirty_ring(struct kvm *kvm, unsigned long pgoff)
>   2928	{
>   2929		if (!KVM_DIRTY_LOG_PAGE_OFFSET)
>   2930			return false;

[1]

>   2931	
> > 2932		return (pgoff >= KVM_DIRTY_LOG_PAGE_OFFSET) &&
>   2933		    (pgoff < KVM_DIRTY_LOG_PAGE_OFFSET +
>   2934		     kvm->dirty_ring_size / PAGE_SIZE);
>   2935	}
>   2936	


-- 
Peter Xu