linux-kernel - Re: [tip:ras/core] x86/RAS: Simplify SMCA HWID descriptor struct

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Date:   Thu, 10 Nov 2016 12:50:04 -0500
From:   Yazen Ghannam <yazen.ghannam@....com>
To:     <mingo@...nel.org>, <tglx@...utronix.de>, <bp@...e.de>,
        <hpa@...or.com>, <linux-kernel@...r.kernel.org>
CC:     <linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:ras/core] x86/RAS: Simplify SMCA HWID descriptor struct

>  static void get_smca_bank_info(unsigned int bank)
>  {
>  	unsigned int i, hwid_mcatype, cpu = smp_processor_id();
> -	struct smca_hwid_mcatype *type;
> +	struct smca_hwid *s_hwid;
>  	u32 high, instance_id;
> -	u16 hwid, mcatype;
>  
>  	/* Collect bank_info using CPU 0 for now. */
>  	if (cpu)
> @@ -162,14 +157,13 @@ static void get_smca_bank_info(unsigned int bank)
>  		return;
>  	}
>  
> -	hwid = high & MCI_IPID_HWID;
> -	mcatype = (high & MCI_IPID_MCATYPE) >> 16;
> -	hwid_mcatype = HWID_MCATYPE(hwid, mcatype);
> +	hwid_mcatype = HWID_MCATYPE(high & MCI_IPID_HWID,
> +				    (high & MCI_IPID_MCATYPE) >> 16);
>

Sorry for catching this late, but it seems this change doesn't compile
correctly. This causes the value of hwid_mcatype to be incorrect, so we
will never match a bank to its type.

I see this with GCC 4.8.5 and 5.4.0. 

There are no warnings or issues when building or booting just
that the behavior is incorrect. 

Disassembly of above change:
      db:       8b 45 e0                mov    -0x20(%rbp),%eax
      de:       41 89 c4                mov    %eax,%r12d
      e1:       25 00 00 ff 0f          and    $0xfff0000,%eax
      e6:       41 c1 ec 10             shr    $0x10,%r12d
      ea:       41 09 c4                or     %eax,%r12d

Disassembly of original code:
     286:       8b 45 d0                mov    -0x30(%rbp),%eax
     289:       41 89 c5                mov    %eax,%r13d
     28c:       c1 e8 10                shr    $0x10,%eax
     28f:       41 81 e5 ff 0f 00 00    and    $0xfff,%r13d
     296:       41 c1 e5 10             shl    $0x10,%r13d
     29a:       41 09 c5                or     %eax,%r13d

Adding extra parentheses in HWID_MCATYPE() gives the same assembly as the
original code and fixes the behavior.

> +	hwid_mcatype = HWID_MCATYPE((high & MCI_IPID_HWID)),
> +				    ((high & MCI_IPID_MCATYPE) >> 16));

Thanks,
Yazen