linux-kernel - Re: /dev/mem and PCI memory = EFAULT (regression?)

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Message-ID: <89281ddd-ea21-6de8-b4dd-57ff4b0d2040@list.ru>
Date:   Sat, 29 Oct 2016 00:36:49 +0300
From:   Stas Sergeev <stsp@...t.ru>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linux kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        dosemu-devel <dosemu-devel@...ts.sourceforge.net>,
        Bart Oldeman <bartoldeman@...rs.sourceforge.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: /dev/mem and PCI memory = EFAULT (regression?)

OK, thanks for the prompt reply, Andy!
I'll try your sysfs suggestion.

Let me just add a bit of CCs to get more people involved.
I believe the ram size involvement makes this smell like a bug,
plus it looks like a regression. Not that I am going to suffer
much (this pass-through stuff always breaks in a million of
different ways, I'll probably just remove it), but its still a very
questionable change IMO.

29.10.2016 00:26, Andy Lutomirski пишет:
> On Fri, Oct 28, 2016 at 2:18 PM, Stas Sergeev <stsp@...t.ru> wrote:
>> 29.10.2016 00:05, Andy Lutomirski пишет:
>>
>>> On Fri, Oct 28, 2016 at 2:03 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>> 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?
>>>>
>>> What is DOSEMU trying to do here?  Access the framebuffer?
>>>
>>> ISTM it would be better to use the DRM or FB layer directly (just map
>>> the framebuffer itself) or, if necessary, use VFIO.
>> Yes, framebuffer.
>> Mapping fb directly is not really an option because dosemu does
>> its own modesetting when you do vga pass-through. So it is
>> usually started that way with "nomodeset=1" and w/o fb.
>> Yes, some crazy people try the pass-through even out of fb
>> console, but that's weird (the problem is most SDL2 builds do
>> not have directfb backend compiled in, otherwise we could
>> just use SDL rendering on top of fb).
>>
>> The thing is, I needed (for testing purposes, unrelated to dosemu)
>> some quick way to access the PCI memory space, and to my surprise I
>> couldn't do that with /dev/mem. Was this really disallowed intentionally?
> I believe so.
>
> Try the /sys/devices/.../resource? and resource?_wc files.
>
> --Andy
>