Re: [PATCH] KVM: VMX: Eliminate warnings by sparse in vmcs12.c and hyperv_evmcs.c

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 10, 2026, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
> 
> Make ROL16() explicitly mask instead of the truncation by u16 cast.
> Sparse complains as follows for vmcs12.c and hyperv_evmcs.c.
>   > vmcs12.c:17:9: warning: cast truncates bits from constant value
>   > (20002 becomes 2)
> 
> It is because ROL16(val, n) includes "(u16)(val) << (n)" whose type is int,
> not u16, due to integer promotion even with a u16 cast.  The outermost u16
> cast in ROL16() may truncate it.  Explicitly mask it to avoid truncation by
> the u16 cast.

Honestly, I'm inclined to ignore sparse.  The code is correct as-is, and adding
the GENMASK just makes it less readable IMO.

> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202603061942.wdSsERnL-lkp@intel.com/
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
>  arch/x86/kvm/vmx/vmcs.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
> index 1f16ddeae9cb..b5b5f4870655 100644
> --- a/arch/x86/kvm/vmx/vmcs.h
> +++ b/arch/x86/kvm/vmx/vmcs.h
> @@ -18,7 +18,8 @@
>   * wasting too much memory, while the "algorithm" is fast enough to be used to
>   * lookup vmcs12 fields on-demand, e.g. for emulation.
>   */
> -#define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 - (n)))))
> +#define ROL16(val, n) ((u16)((((u16)(val) << (n)) & GENMASK_U16(15, 0)) | \
> +			     ((u16)(val) >> (16 - (n)))))
>  #define VMCS12_IDX_TO_ENC(idx) ROL16(idx, 10)
>  #define ENC_TO_VMCS12_IDX(enc) ROL16(enc, 6)
>  
> 
> base-commit: 5128b972fb2801ad9aca54d990a75611ab5283a9
> -- 
> 2.45.2
>