linux-kernel - Re: [RFC][PATCH 2/2] x86/retpoline: Compress retpolines

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Date:   Fri, 19 Feb 2021 08:14:39 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, tony.luck@...el.com, pjt@...gle.com,
        linux-kernel@...r.kernel.org, r.marek@...embler.cz,
        jpoimboe@...hat.com, jikos@...nel.org
Subject: Re: [RFC][PATCH 2/2] x86/retpoline: Compress retpolines

On Thu, Feb 18, 2021 at 05:59:40PM +0100, Peter Zijlstra wrote:
> By using int3 as a speculation fence instead of lfence, we can shrink
> the longest alternative to just 15 bytes:
> 
>   0:   e8 05 00 00 00          callq  a <.altinstr_replacement+0xa>
>   5:   f3 90                   pause  
>   7:   cc                      int3   
>   8:   eb fb                   jmp    5 <.altinstr_replacement+0x5>
>   a:   48 89 04 24             mov    %rax,(%rsp)
>   e:   c3                      retq   
> 
> This means we can change the alignment from 32 to 16 bytes and get 4
> retpolines per cacheline, $I win.

You mean I$ :)

In any case, for both:

Reviewed-by: Borislav Petkov <bp@...e.de>

and it looks real nice here, the size:

 readelf -s vmlinux | grep __x86_indirect
 78966: ffffffff81c023e0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 81653: ffffffff81c02390    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 82338: ffffffff81c02430    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 82955: ffffffff81c02380    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 85057: ffffffff81c023f0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 89996: ffffffff81c023a0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 91094: ffffffff81c02400    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 91278: ffffffff81c023b0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 92015: ffffffff81c02360    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 92722: ffffffff81c023c0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 97062: ffffffff81c02410    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 98687: ffffffff81c023d0    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 99076: ffffffff81c02350    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
 99500: ffffffff81c02370    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]
100579: ffffffff81c02420    15 FUNC    GLOBAL DEFAULT    1 __x86_indirect_t[...]

-- 
Regards/Gruss,
    Boris.

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