Re: [RFC] prevent "dd if=/dev/mem" crash

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> 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).

	--david