Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 20 Jun 2019 09:15:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc:     Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>,
        "linux-snps-arc@lists.infradead.org" 
        <linux-snps-arc@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@synopsys.com>,
        Jason Baron <jbaron@akamai.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: Re: [PATCH] ARC: ARCv2: jump label: implement jump label patching
Message-ID: <20190620071514.GR3419@hirez.programming.kicks-ass.net>
References: <20190614164049.31626-1-Eugeniy.Paltsev@synopsys.com>
 <C2D7FE5348E1B147BCA15975FBA2307501A252CCC3@us01wembx1.internal.synopsys.com>
 <20190619081227.GL3419@hirez.programming.kicks-ass.net>
 <C2D7FE5348E1B147BCA15975FBA2307501A252E40B@us01wembx1.internal.synopsys.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <C2D7FE5348E1B147BCA15975FBA2307501A252E40B@us01wembx1.internal.synopsys.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jun 19, 2019 at 11:55:41PM +0000, Vineet Gupta wrote:

> FWIW I tried to avoid all of this by using the 2 byte NOP_S and B_S variants which
> ensures we can never straddle cache line so the alignment issue goes away. There's
> a nice code size reduction too - see [1] . But I get build link errors in
> networking code around DO_ONCE where the unlikely code is too much and offset
> can't be encoded in signed 10 bits which B_S is allowed.

Yeah, so on x86 we have a 2 byte and a 5 byte relative jump and have the
exact same issue. We're currently using 5 byte jumps unconditionally for
the same reason.

Getting it to use the 2 byte one where possible is a 'fun' project for
someone with spare time at some point.  It might need a GCC plugin to
pull off, I've not put too much tought into it.