From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Mon, 3 Feb 2014 16:36:09 +0000
Subject: [PATCH resend 1/2] arm64: defer reloading a task's FPSIMD state
 to userland resume
In-Reply-To: <1391163196-27619-1-git-send-email-ard.biesheuvel@linaro.org>
References: <1391163196-27619-1-git-send-email-ard.biesheuvel@linaro.org>
Message-ID: <20140203163609.GK14112@mudshark.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Hi Ard,

On Fri, Jan 31, 2014 at 10:13:15AM +0000, Ard Biesheuvel wrote:
> If a task gets scheduled out and back in again and nothing has touched
> its FPSIMD state in the mean time, there is really no reason to reload
> it from memory. Similarly, repeated calls to kernel_neon_begin() and
> kernel_neon_end() will preserve and restore the FPSIMD state every time.
> 
> This patch defers the FPSIMD state restore to the last possible moment,
> i.e., right before the task re-enters userland. If a task does not enter
> userland at all (for any reason), the existing FPSIMD state is preserved
> and may be reused by the owning task if it gets scheduled in again on the
> same CPU.

The one situation I'm unsure of here is how you deal with the saved fpsimd
state potentially being updated by a signal handler or a debugger. In this
case, we probably need to set _TIF_FOREIGN_FPSTATE to force a reload, or are
you handling this some other way?

Will