linux-kernel - Re: [patch 1/4] x86: vSMP: Fix is_vsmp

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Date:	Thu, 20 Mar 2008 11:40:46 -0700
From:	Ravikiran G Thirumalai <kiran@...lex86.org>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Andi Kleen <ak@...e.de>, shai@...lex86.org
Subject: Re: [patch 1/4] x86: vSMP: Fix is_vsmp_box()

On Thu, Mar 20, 2008 at 12:44:37AM -0700, Yinghai Lu wrote:
>On Thu, Mar 20, 2008 at 12:39 AM, Ravikiran G Thirumalai
><kiran@...lex86.org> wrote:
>> is_vsmp_box() currently does not work on vSMPowered systems,  as pci cfg
>>  space is not read correctly -- This patch fixes it.
>>
>>  Signed-off-by: Ravikiran Thirumalai <kiran@...lex86.org>
>>
>>  Index: linux.git.trees/arch/x86/kernel/vsmp_64.c
>>  ===================================================================
>>  --- linux.git.trees.orig/arch/x86/kernel/vsmp_64.c      2008-03-19 13:30:35.116766719 -0700
>>  +++ linux.git.trees/arch/x86/kernel/vsmp_64.c   2008-03-19 13:39:20.074685590 -0700
>>  @@ -84,8 +84,10 @@ int is_vsmp_box(void)
>>                 return vsmp;
>>
>>         /* Check if we are running on a ScaleMP vSMP box */
>>  -       if (read_pci_config(0, 0x1f, 0, PCI_VENDOR_ID) ==
>>  -            (PCI_VENDOR_ID_SCALEMP || (PCI_DEVICE_ID_SCALEMP_VSMP_CTL << 16)))
>>  +       if ((read_pci_config_16(0, 0x1f, 0, PCI_VENDOR_ID) ==
>>  +            PCI_VENDOR_ID_SCALEMP) &&
>>  +           (read_pci_config_16(0, 0x1f, 0, PCI_DEVICE_ID) ==
>>  +           PCI_DEVICE_ID_SCALEMP_VSMP_CTL))
>>                 vsmp = 1;
>>
>>         return vsmp;
>
>why read two times
>

Well, the pci cfg space read happens just _once_ during the boot, as
the result is cached in a static flag. The above code is better readable.
So readability is better than micro-optimization here.

Thanks,
Kiran
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