From owner-freebsd-hackers Thu Jan 20 8:45: 1 2000 Delivered-To: freebsd-hackers@freebsd.org Received: from c62443-a.frmt1.sfba.home.com (c62443-a.frmt1.sfba.home.com [24.0.69.165]) by hub.freebsd.org (Postfix) with ESMTP id 77A8A14CEC for ; Thu, 20 Jan 2000 08:44:59 -0800 (PST) (envelope-from adsharma@c62443-a.frmt1.sfba.home.com) Received: (from adsharma@localhost) by c62443-a.frmt1.sfba.home.com (8.9.3/8.9.3) id IAA19677; Thu, 20 Jan 2000 08:44:57 -0800 Date: Thu, 20 Jan 2000 08:44:57 -0800 From: Arun Sharma To: Zhihui Zhang Cc: hackers@freebsd.org Subject: Re: Accessing user data from kernel Message-ID: <20000120084457.A19569@sharmas.dhs.org> References: <200001200214.SAA17214@c62443-a.frmt1.sfba.home.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.95.6i In-Reply-To: ; from Zhihui Zhang on Thu, Jan 20, 2000 at 10:04:16AM -0500 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Thu, Jan 20, 2000 at 10:04:16AM -0500, Zhihui Zhang wrote: > Point 2 seems to be saying that we would rather sacrifice some performance > to gain a cleaner interface (people are talking about eliminating kernel > copying for a long time). Consider the physical I/O on a raw device, where > we map the user data again in the KVA without copying the data. Why do we > do this double mapping, when we can access the user data directly? > Direct I/O to user space should be treated as an optimization. Such I/O requires wiring down all the user pages before I/O can happen. Hence it requires special previleges. Why does it get mapped to KVA ? Because of point 1. -Arun To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message