On Thu, Jan 31, 2019 at 12:17:08PM -0800, Andrey Smirnov wrote:
> On Thu, Jan 31, 2019 at 2:55 AM Peter Mamonov <pmamo...@gmail.com> wrote:
> >
> > Hello, Andrey,
> >
> > > In order to allow access to second half of address space on 64-bit
> > > machines, add code that creates /dev/highmem dedicated for that.
> > >
> > > Note that due to maximum file size being limited to MAX_LFS_FILESIZE
> > > or 0x7fff_ffff_ffff_ffff bytes at addresses 0x7fff_ffff_ffff_ffff and
> > > 0xffff_ffff_ffff_ffff cannot be access through /dev/mem and
> > > /dev/hightmem correspondingly.
> >
> > Does it imply using `-s /dev/highmem` argument when accessing addresses
> > beyond
> > MAX_LFS_FILESIZE?
>
> Yes, it does.
>
> > That's not very convenient:
> >
> > $ git grep -l /dev/mem
> > commands/crc.c
> > commands/digest.c
> > commands/disasm.c
> > commands/md.c
> > commands/memcmp.c
> > commands/memcpy.c
> > commands/memset.c
> > commands/mm.c
> > commands/mw.c
> >
>
> I don't disagree, but there isn't much that can be done about that
> AFAICT. The first version of this fix kept single /dev/mem file, but
> ended up introducing quite a bit of special cases all over the FS
> layer, so it was rejected. The only two options to make it more
> convenient that I can see would be to either modify "/dev/mem" users
> to make "/dev/highmem" substitution automatically based on offset they
> are given, or, assuming it is possible, create a mapping that would
> expose all of the interesting registers on MIPS64 in the lower half of
> 64-bit address space.
Guess we can just adopt Linux's approach. Linux recognizes /dev/mem's lseek as
a special case and has a dedicated implementation for it. Here is a relevant
comment from drivers/char/mem.c:
/*
* The memory devices use the full 32/64 bits of the offset, and so we
cannot
* check against negative addresses: they are ok. The return value is
weird,
* though, in that case (0).
*
* also note that seeking relative to the "end of file" isn't supported:
* it has no meaning, so it returns -EINVAL.
*/
static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
{
Regards,
Peter
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