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Date:   Tue, 25 Aug 2020 20:34:02 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Yazen Ghannam <yazen.ghannam@amd.com>
Cc:     "Luck, Tony" <tony.luck@intel.com>,
        "linux-edac@vger.kernel.org" <linux-edac@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "Smita.KoralahalliChannabasappa@amd.com" 
        <Smita.KoralahalliChannabasappa@amd.com>
Subject: Re: [PATCH] x86/mce: Increase maximum number of banks to 64
Message-ID: <20200825183402.GG12107@zn.tnic>
References: <20200820170624.1855825-1-Yazen.Ghannam@amd.com>
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 <20200820180054.GA2033274@yaz-nikka.amd.com>
 <6ecac9c40d7b4491b9a87c8927a4aca6@intel.com>
 <20200824151105.GA3478291@yaz-nikka.amd.com>
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On Mon, Aug 24, 2020 at 10:11:05AM -0500, Yazen Ghannam wrote:
> Thanks. Yeah, they're identical:
>    text    data     bss     dec     hex filename
>    15710076        13519306        5398528 34627910        2106146   vmlinux-32banks
>    15710076        13519306        5398528 34627910        2106146   vmlinux-64banks

So I went crazy here and built allmodconfigs for hypothetical CPUs which
support up to 4K MCA banks:

   text    data     bss     dec     hex filename
24618040        27211906        35409988        87239934        5332cfe vmlinux-32banks
24618040        27211906        35409988        87239934        5332cfe vmlinux-64banks
24618040        27211906        35409988        87239934        5332cfe vmlinux-128banks

For the first three - up to 128 banks, the sizes don't change.

24618000        27216002        35401796        87235798        5331cd6 vmlinux-256banks

Somewhere after 128 banks, it grows the data section but by exactly 4K,
i.e., one more page:

27216002 - 27211906 = 4096

The last page is not full, of course.

27216002 % 4096 = 2178
27211906 % 4096 = 2178

24618000        27220098        35401796        87239894        5332cd6 vmlinux-512banks

The same happens here.

27220098 - 27216002 = 4096
27220098 % 4096 = 2178

And so on:

24618000        27228290        35393604        87239894        5332cd6 vmlinux-1024banks
24618000        27244674        35377220        87239894        5332cd6 vmlinux-2048banks
24618000        27277442        35344452        87239894        5332cd6 vmlinux-4Kbanks

So what I think happens is, the first 128 banks fit within the same
.data section page thus vmlinux doesn't increase.

I guess there might be a .config which could cause that data increase
when the 32->64 banks move won't fit in the last .data page. It depends
on where those ~2K you computed and I snipped, land.

But since most kernels are allmodconfig builds, I guess we should be
fine.

So yeah, I guess we can take this one after you summarize the findings
of this thread in v2's commit message. :-)

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette