From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752796AbcJCUFG (ORCPT <rfc822;w@1wt.eu>);
        Mon, 3 Oct 2016 16:05:06 -0400
Received: from mga07.intel.com ([134.134.136.100]:40002 "EHLO mga07.intel.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1752136AbcJCUE7 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 3 Oct 2016 16:04:59 -0400
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.31,291,1473145200"; 
   d="scan'208";a="1039821954"
Subject: Re: [PATCH RFC 4/5] x86,fpu: lazily skip FPU restore when still
 loaded
To: riel@redhat.com, linux-kernel@vger.kernel.org
References: <1475353895-22175-1-git-send-email-riel@redhat.com>
 <1475353895-22175-5-git-send-email-riel@redhat.com>
Cc: x86@kernel.org, tglx@linutronix.de, pbonzini@redhat.com,
        mingo@redhat.com, luto@kernel.org, hpa@zytor.com, bp@suse.de
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <57F2B9E9.7030806@linux.intel.com>
Date: Mon, 3 Oct 2016 13:04:57 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.8.0
MIME-Version: 1.0
In-Reply-To: <1475353895-22175-5-git-send-email-riel@redhat.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 10/01/2016 01:31 PM, riel@redhat.com wrote:
>  /*
> + * Check whether an FPU's register set is still loaded in the CPU.
> + */
> +static inline bool fpu_lazy_skip_restore(struct fpu *fpu)
> +{
> +	bool still_loaded = (fpu->fpstate_active &&
> +			     fpu->last_cpu == raw_smp_processor_id() &&
> +			     __this_cpu_read(fpu_fpregs_owner_ctx) == fpu);
> +
> +	fpu->fpregs_active = still_loaded;
> +	return still_loaded;
> +}

I wonder if we should call this something more along the lines of
fpregs_activate_fast(), which returns if it managed to do the activation
fast or not.  I _think_ that's more along the lines of what it is
actually doing.  The fact that it can be lazy is really an
implementation detail.

What are the preempt rules with this thing?  This needs to be called in
preempt-disabled contexts, right?