linux-kernel - Re: arch/x86/kvm/vmx/hyperv.h:79:30: sparse: sparse: cast truncates bits from constant value (1b009b becomes 9b)

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Message-ID: <87v89jmc3q.fsf@redhat.com>
Date:   Thu, 30 Nov 2023 17:37:13 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kernel test robot <lkp@...el.com>
Cc:     oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org
Subject: Re: arch/x86/kvm/vmx/hyperv.h:79:30: sparse: sparse: cast truncates
 bits from constant value (1b009b becomes 9b)

kernel test robot <lkp@...el.com> writes:

> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   3b47bc037bd44f142ac09848e8d3ecccc726be99
> commit: a789aeba419647c44d7e7320de20fea037c211d0 KVM: VMX: Rename "vmx/evmcs.{ch}" to "vmx/hyperv.{ch}"
> date:   1 year ago
> config: x86_64-randconfig-123-20231130 (https://download.01.org/0day-ci/archive/20231130/202311302231.sinLrAig-lkp@intel.com/config)
> compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.git ae42196bc493ffe877a7e3dff8be32035dea4d07)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231130/202311302231.sinLrAig-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@...el.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202311302231.sinLrAig-lkp@intel.com/
>
> sparse warnings: (new ones prefixed by >>)
>    arch/x86/kvm/vmx/hyperv.h:79:30: sparse: sparse: cast truncates bits from constant value (a000a becomes a)

This is what ROL16() macro does but the thing is: we actually want to
truncate bits by doing an explicit (u16) cast. We can probably replace
this with '& 0xffff':

#define ROL16(val, n) ((((u16)(val) << (n)) | ((u16)(val) >> (16 - (n)))) & 0xffff)

but honestly I don't see much point...

-- 
Vitaly