/dev/mem and PCI memory = EFAULT?

From	Stas Sergeev <stsp@list.ru>
Newsgroups	linux.kernel
Subject	/dev/mem and PCI memory = EFAULT?
Date	2016-10-28 23:10 +0200
Message-ID	<sxmzv-59w-7@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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Hello.

For the long time dosemu used /dev/mem for vga pass-through.
Now it appears /dev/mem has this check:
http://lxr.free-electrons.com/source/drivers/char/mem.c#L51
which prevents an accesses to PCI memory regions if the
"high_memory" points low enough. It seems "high_memory"
just points to the end of the physical ram, so depending on
the ram size you either can access PCI devices or you get
EFAULT.
Was it wrong to use /dev/mem for accessing the PCI devices?
How should I do that now?

Note: I haven't properly tested that case. I can see the problem
only under qemu, because all my machines has enough ram for
the things to work. Still the failure under qemu makes me wonder
if I should stop using /dev/mem for accessing PCI memory.

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/dev/mem and PCI memory = EFAULT? Stas Sergeev <stsp@list.ru> - 2016-10-28 23:10 +0200
  Re: /dev/mem and PCI memory = EFAULT? Andy Lutomirski <luto@amacapital.net> - 2016-10-28 23:10 +0200
    Re: /dev/mem and PCI memory = EFAULT? Stas Sergeev <stsp@list.ru> - 2016-10-28 23:20 +0200
      Re: /dev/mem and PCI memory = EFAULT? Andy Lutomirski <luto@amacapital.net> - 2016-10-28 23:30 +0200
        Re: /dev/mem and PCI memory = EFAULT (regression?) Stas Sergeev <stsp@list.ru> - 2016-10-29 00:10 +0200
          Re: /dev/mem and PCI memory = EFAULT (regression?) Linus Torvalds <torvalds@linux-foundation.org> - 2016-10-29 01:30 +0200
            Re: /dev/mem and PCI memory = EFAULT (regression?) Stas Sergeev <stsp@list.ru> - 2016-10-29 01:50 +0200

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