Messages in this thread | | | From | David Mosberger <> | Date | Fri, 17 Oct 2003 18:48:33 -0700 | Subject | Re: [RFC] prevent "dd if=/dev/mem" crash |
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>>>>> On Fri, 17 Oct 2003 17:49:55 -0700, Andrew Morton <akpm@osdl.org> 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).
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