From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Mosberger Date: Sat, 18 Oct 2003 01:48:33 +0000 Subject: Re: [RFC] prevent "dd if=/dev/mem" crash Message-Id: List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org >>>>> On Fri, 17 Oct 2003 17:49:55 -0700, Andrew Morton said: Andrew> We _want_ to be able to read mmio ranges via /dev/mem, don't Andrew> we? I guess it has never come up because everyone uses Andrew> kmem. I just don't see how making a "dd if=/dev/mem" safe and allowing access to arbitrary physical memory can go to together. Given that /dev/mem _is_ being used for accessing mmio space, is it really worth bothering trying to make such a "dd" safe? Andrew> If the hardware doesn't give the system programmer a choice Andrew> then the hardware is poorly designed, surely? Emh, we're talking about _physical_ memory accesses here. AFAIK, failures on physical memory accesses are never signaled with synchronous faults (not on any reasonably modern high performance architecture, at least). Loads probably _could_ be signalled synchronously, but consider stores: would you really want to wait with retiring a store until it has made it all the way to some slow ISA device? I think not (IN/OUT do that). No, modern CPUs check the TLB/page-table and if that check passes, they'll _assume_ the memory access will complete without errors. If it doesn't, they signal an asynchronous failure (e.g., via an MCA). --david