Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitraryphysical addresses

[Cong Wang]

ä 2011å06æ20æ 21:24, Dave Anderson åé:
> > On Mon, Jun 20, 2011 at 10:42:47, Amerigo Wang<amwang@xxxxxxxxxx>  wrote:
> > > On Fri, Jun 17, 2011 at 5:55 PM, Petr Tesarik<ptesarik@xxxxxxx>  wrote:
> > > Dne PÃÂ 17. ÃÂervna 2011 11:30:32 Ingo Molnar napsal(a):
> > > > * Petr Tesarik<ptesarik@xxxxxxx>  wrote:
> > > > > This patch series enhances /dev/mem, so that read and write is
> > > > > possible at any address. The patchset includes actual
> > > > > implementation for x86.
> > > >
> > > > This series lacks a description of why this is desired.
> > >
> > > Hi Ingo,
> > >
> > > > My strong opinion is that it's not desired at all: /dev/mem never
> > > > worked beyond 4G addresses so by today it has become largely obsolete
> > > > and is on the way out really.
> > > >
> > > > I'm aware of these current /dev/mem uses:
> > > >
> > > >   - Xorg maps below 4G non-RAM addresses and the video BIOS
> > > >
> > > >   - It used to have some debugging role but these days kexec and kgdb
> > > >     has largely taken over that role - partly due to the 4G limit.
> > >
> > > It is still used as a "memory source" by Dave Anderson's crash utility for
> > > live examination of a running system. Redhat has "overcome" the /dev/mem
> > > deficiencies by writing an out-of-tree re-implementation of /dev/mem, which
> > > uses /dev/crash instead. As it is an "unnecessary duplication of an existing
> > > driver", this method was rejected by the project manager here at SUSE.
> > >
> > > The suggested alternative was to enhance (or fix) the existing driver. Without
> > > this patch series there is no way to access high memory. In conjunction with
> > > CONFIG_HIGHPTE, it makes the crash utility near to useless on anything with
> > > high memory, because crash can no longer translate virtual to physical
> > > addresses.
> >
> > How about /proc/kcore? AFAIK, it can access highmem, but Dave didn't consider
> > it for some reason.
>
> I don't know what you mean by I "didn't consider it", because
> the crash utility does support using /proc/kcore as an alternative
> live memory source.

Sorry, I recall that in our last discussion you didn't
mention this. But it is good to know that crash supports this!

> The problem is that /proc/kcore can only access highmem if it
> is mapped as kernel virtual address.  So it cannot read 32-bit
> highmem PTE's, user-space memory, etc.

Hmm, looking at the code, you are right, it only has
low memory in kernel space.

But what's the problem of adding highmem into kcore? :-/

Thanks.
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