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:   Mon, 19 Mar 2018 16:05:23 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     David Laight <David.Laight@ACULAB.COM>
cc:     'Rahul Lakkireddy' <rahul.lakkireddy@chelsio.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "ganeshgr@chelsio.com" <ganeshgr@chelsio.com>,
        "nirranjan@chelsio.com" <nirranjan@chelsio.com>,
        "indranil@chelsio.com" <indranil@chelsio.com>
Subject: RE: [RFC PATCH 0/3] kernel: add support for 256-bit IO access
In-Reply-To: <7f0ddb3678814c7bab180714437795e0@AcuMS.aculab.com>
Message-ID: <alpine.DEB.2.21.1803191557400.2010@nanos.tec.linutronix.de>
References: <cover.1521469118.git.rahul.lakkireddy@chelsio.com> <7f0ddb3678814c7bab180714437795e0@AcuMS.aculab.com>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, 19 Mar 2018, David Laight wrote:
> From: Rahul Lakkireddy
> In principle it ought to be possible to get access to one or two
> (eg) AVX registers by saving them to stack and telling the fpu
> save code where you've put them.

No. We have functions for this and we are not adding new ad hoc magic.

> OTOH, for x86, if the code always runs in process context (eg from a
> system call) then, since the ABI defines them all as caller-saved
> the AVX(2) registers, it is only necessary to ensure that the current
> FPU registers belong to the current process once.
> The registers can be set to zero by an 'invalidate' instruction on
> system call entry (hope this is done) and after use.

Why would a system call touch the FPU registers? The kernel normally does
not use FPU instructions and the code which explicitely does has to take
care of save/restore. It would be performance madness to fiddle with the
FPU stuff unconditionally if nothing uses it.

Thanks,

	tglx