linux-kernel - Re: [PATCH v4 6/9] x86: prevent inline distortion by paravirt ops

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Date:   Tue, 19 Jun 2018 17:09:22 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Nadav Amit <namit@...are.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Alok Kataria <akataria@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 6/9] x86: prevent inline distortion by paravirt ops

On 12/06/18 13:50, Nadav Amit wrote:
> GCC considers the number of statements in inlined assembly blocks,
> according to new-lines and semicolons, as an indication to the cost of
> the block in time and space. This data is distorted by the kernel code,
> which puts information in alternative sections. As a result, the
> compiler may perform incorrect inlining and branch optimizations.
> 
> The solution is to set an assembly macro and call it from the inlined
> assembly block. As a result GCC considers the inline assembly block as
> a single instruction.
> 
> The effect of the patch is a more aggressive inlining, which also
> causes a size increase of kernel.
> 
>    text	   data	    bss	    dec	    hex	filename
> 18147336 10226688 2957312 31331336 1de1408 ./vmlinux before
> 18162555 10226288 2957312 31346155 1de4deb ./vmlinux after (+14819)
> 
> Static text symbols:
> Before:	40053
> After:	39942	(-111)
> 
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Alok Kataria <akataria@...are.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: virtualization@...ts.linux-foundation.org
> 
> Signed-off-by: Nadav Amit <namit@...are.com>

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen