Date: Thu, 20 Jan 2000 08:44:57 -0800 From: Arun Sharma <adsharma@sharmas.dhs.org> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: hackers@freebsd.org Subject: Re: Accessing user data from kernel Message-ID: <20000120084457.A19569@sharmas.dhs.org> In-Reply-To: <Pine.GSO.3.96.1000120095742.24022A-100000@sol.cs.binghamton.edu>; from Zhihui Zhang on Thu, Jan 20, 2000 at 10:04:16AM -0500 References: <200001200214.SAA17214@c62443-a.frmt1.sfba.home.com> <Pine.GSO.3.96.1000120095742.24022A-100000@sol.cs.binghamton.edu>
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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
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