From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Mon, 22 Feb 2016 16:52:09 +0000
Subject: [PATCH v3 4/4] arm64: prevent __va() translations before
 memstart_addr is assigned
In-Reply-To: <1455289046-21321-5-git-send-email-ard.biesheuvel@linaro.org>
References: <1455289046-21321-1-git-send-email-ard.biesheuvel@linaro.org>
 <1455289046-21321-5-git-send-email-ard.biesheuvel@linaro.org>
Message-ID: <20160222165209.GK31168@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Fri, Feb 12, 2016 at 03:57:26PM +0100, Ard Biesheuvel wrote:
> Since memstart_addr is assigned relatively late in the boot code,
> after generic code like DT parsing and memblock manipulation has
> already occurred, we need to ensure that no __va() translation occur
> until memstart_addr has been set to a meaningful value.
> 
> So initialize memstart_addr to a value that cannot represent a valid
> physical address, and BUG() if memstart_addr is referenced while it
> still holds this value. Note that the > comparison against LLONG_MAX
> (not ULLONG_MAX) resolves to a single tbnz instruction that performs
> a conditional jump to a brk instruction that is emitted out of line.

Even so, I'd imagine that having a measurable impact on system
performance. Did you have a go at benchmarking this?

Will