linux-kernel - Re: [PATCH] x86/apic: Use byte array apic_version[], not int array. Saves up to 96k

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Date:   Tue, 13 Sep 2016 20:07:29 +0200
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        "H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Mike Travis <travis@....com>
Subject: Re: [PATCH] x86/apic: Use byte array apic_version[], not int array.
 Saves up to 96k



On 09/13/2016 05:33 PM, Thomas Gleixner wrote:
> On Sun, 11 Sep 2016, Borislav Petkov wrote:
>> On Fri, Sep 09, 2016 at 10:32:04AM +0200, Denys Vlasenko wrote:
>>> This array is [MAX_LOCAL_APIC], and MAX_LOCAL_APIC can easily be up to 32k.
>>>
>>> This patch changes apic_version[] array elements from int to u8 -
>>> APIC version values as of year 2016 are no larger than 0x1f on all known CPUs.
>>> Version field in the APIC register is 8 bit wide - not likely
>>> to overflow byte range in foreseeable future.
>>>
>>> The "ver" argument of generic_processor_info(id,ver), which goes into apic_version[id],
>>> is also changed from int to u8: make it obvious that assignment can't overflow.
>>>
>>> generic_processor_info() has four callsites, none of them can put an out-of-range value
>>> into this argument.
>>
>> Right, so I dug a bit into this and found:
>>
>> http://marc.info/?l=linux-kernel&m=123230551709711
>>
>> and
>>
>> b2b815d80a5c ("x86: put trigger in to detect mismatched apic versions")
>>
>> It is from 2009 and I don't know how relevant 16-bit APIC IDs are
>> anymore... I guess you probably want to run this by SGI folk first.
>>
>> Otherwise I was going to propose to kill that apic_version array
>> altogether and cache only the version of the previous CPU and compare it
>> to the current one to catch mismatches...
>
> Yeah, the idea was back then to eliminate the array, but we wanted to make
> sure that we don't have systems out in the wild which have different apic
> versions. I really doubt that we can deal with that proper, so having a
> single version entry and yelling loudly when we detect a mismatch is good
> enough.

Makes sense. I'll send a patch