linux-kernel - Re: [PATCH] x86: skip check for spurious faults for non-present faults

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Message-ID: <2edd49ee-077e-4126-b09a-077b7f6be553@email.android.com>
Date:	Thu, 15 May 2014 14:20:15 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave.hansen@...el.com>,
	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH] x86: skip check for spurious faults for non-present faults

I'm trying to wrap my head around any forward compatibility concerns... if we misidentify a fault as spurious that would be bad.  

On May 15, 2014 1:50:13 PM PDT, Dave Hansen <dave.hansen@...el.com> wrote:
>On 05/12/2014 03:29 AM, David Vrabel wrote:
>> -	/* Reserved-bit violation or user access to kernel space? */
>> -	if (error_code & (PF_USER | PF_RSVD))
>> +	/* Only check for spurious faults on supervisor write or
>> +	   instruction faults. */
>> +	if (error_code != (PF_WRITE | PF_PROT)
>> +	    && error_code != (PF_INSTR | PF_PROT))
>>  		return 0;
>
>This changes the semantics a bit too much for me to feel happy about
>it.
> This is at best missing quite a bit of detail from the changelog.
>
> 1. 'return 0' means "this was not a spurious fault"
> 2. We used to check for the presence of PF_USER|PF_RSVD
> 3. This patch checks now for two _explicit_ conditions, which
>    implicitly check for the _absence_ of the two bits we checked for
>    before.
>
>I do believe your patch is correct, but it took me a bit to convince
>myself that it was the right thing.  Please be explicit (in the
>comment)
>about the exact PTE transitions that you expect to get you here.
>
>Also, I have to wonder if you can just leave the original if() in
>there.
> You're making this _more_ restrictive than it was before, and I wonder
>if it might just be more clear if you have both checks.  The compiler
>might even compile it down to the same code, just changing the
>immediate
>that was generated for the mask that you're checking.

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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