Messages in this thread | | | Subject | Re: [PATCH] x86/apic: Use byte array apic_version[], not int array. Saves up to 96k | From | Denys Vlasenko <> | Date | Tue, 13 Sep 2016 20:07:29 +0200 |
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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
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