linux-kernel - Re: arch/x86/kvm/mmu/paging_tmpl.h:106:24: sparse: sparse: cast truncates bits from constant value (ffffffffff000 becomes fffff000)

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Date:   Wed, 7 Sep 2022 14:41:22 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     kernel test robot <lkp@...el.com>
Cc:     kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: arch/x86/kvm/mmu/paging_tmpl.h:106:24: sparse: sparse: cast
 truncates bits from constant value (ffffffffff000 becomes fffff000)

On Mon, Sep 05, 2022, kernel test robot wrote:
> Hi Sean,
> 
> FYI, the error/warning was bisected to this commit, please ignore it if it's irrelevant.
> 
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   7e18e42e4b280c85b76967a9106a13ca61c16179
> commit: 70e41c31bc7776b262cd9f524df3dfc2b5869a0a KVM: x86/mmu: Use common logic for computing the 32/64-bit base PA mask
> date:   3 months ago
> config: i386-randconfig-s001-20220905 (https://download.01.org/0day-ci/archive/20220905/202209051509.RDisCzFu-lkp@intel.com/config)
> compiler: gcc-11 (Debian 11.3.0-5) 11.3.0
> reproduce:
>         # apt-get install sparse
>         # sparse version: v0.6.4-39-gce1a6720-dirty
>         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=70e41c31bc7776b262cd9f524df3dfc2b5869a0a
>         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>         git fetch --no-tags linus master
>         git checkout 70e41c31bc7776b262cd9f524df3dfc2b5869a0a
>         # save the config file
>         mkdir build_dir && cp config build_dir/.config
>         make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 SHELL=/bin/bash arch/x86/kvm/
> 
> If you fix the issue, kindly add following tag where applicable
> Reported-by: kernel test robot <lkp@...el.com>
> 
> sparse warnings: (new ones prefixed by >>)
>    arch/x86/kvm/mmu/mmu.c:2550:9: sparse: sparse: context imbalance in 'mmu_try_to_unsync_pages' - different lock contexts for basic block
>    arch/x86/kvm/mmu/mmu.c: note: in included file:
> >> arch/x86/kvm/mmu/paging_tmpl.h:106:24: sparse: sparse: cast truncates bits from constant value (ffffffffff000 becomes fffff000)
>    arch/x86/kvm/mmu/paging_tmpl.h:426:24: sparse: sparse: cast truncates bits from constant value (ffffffffff000 becomes fffff000)
>    arch/x86/kvm/mmu/mmu.c:4618:57: sparse: sparse: cast truncates bits from constant value (ffffff33 becomes 33)
>    arch/x86/kvm/mmu/mmu.c:4620:56: sparse: sparse: cast truncates bits from constant value (ffffff0f becomes f)
>    arch/x86/kvm/mmu/mmu.c:4622:57: sparse: sparse: cast truncates bits from constant value (ffffff55 becomes 55)
> 
> vim +106 arch/x86/kvm/mmu/paging_tmpl.h
> 
> b3fcdb04a98035 arch/x86/kvm/mmu/paging_tmpl.h Sean Christopherson 2022-06-14  103  
> e04da980c35d75 arch/x86/kvm/paging_tmpl.h     Joerg Roedel        2009-07-27  104  static gfn_t gpte_to_gfn_lvl(pt_element_t gpte, int lvl)
> 5fb07ddb183eb4 drivers/kvm/paging_tmpl.h      Avi Kivity          2007-11-21  105  {
> e04da980c35d75 arch/x86/kvm/paging_tmpl.h     Joerg Roedel        2009-07-27 @106  	return (gpte & PT_LVL_ADDR_MASK(lvl)) >> PAGE_SHIFT;

This truncation is expected and intentional in this case (guest is using 32-bit PTEs).

> 5fb07ddb183eb4 drivers/kvm/paging_tmpl.h      Avi Kivity          2007-11-21  107  }
> 5fb07ddb183eb4 drivers/kvm/paging_tmpl.h      Avi Kivity          2007-11-21  108  
> 
> :::::: The code at line 106 was first introduced by commit
> :::::: e04da980c35d75fa050ba4009ad99025432d8d7d KVM: MMU: make page walker aware of mapping levels
> 
> :::::: TO: Joerg Roedel <joerg.roedel@....com>
> :::::: CC: Avi Kivity <avi@...hat.com>
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://01.org/lkp