[Qemu-devel] Bus errors and /dev/shm -- was: KQEMU bus errors

[Darryl Dixon <esrever_otua@pythonhacker.is-a-geek.net>]

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I'm not quite clear on why /dev/shm should affect the 2005-02-11 version
and not the 2005-02-09 version of the code, but I took you at your word
and tried first with qemu guest mem == size of /dev/shm, and once again
received 'Bus error'.  I then set qemu guest mem == 3/4 of /dev/shm and
it seems to be working OK.

So, can anyone explain why the apparent regression, and why /dev/shm is
used at all?  I would be happier if I understood :)

Incidentally, the 'Protection Error' received by Win9x guest appears to
be orthogonal to this problem and isn't fixed by the above like WinXP
is.

Cheers,
D


On Sun, 2005-02-13 at 15:51 +0100, Grzegorz Kulewski wrote:

> On Sun, 13 Feb 2005, Julian Seward wrote:
> 
> > On Sunday 13 February 2005 11:13, Jonas Maebe wrote:
> >> On 13 feb 2005, at 10:42, Darryl Dixon wrote:
> >>> Most of the references that I can find for a Linux 'Bus error' talk
> >>> about unaligned memory accesses.
> >>
> >> Indeed. The only way I know of to get a bus error under Linux/x86 from
> >> a user space program, is to turn on the alignment check flag in the
> >> eflags register, followed by an unaligned memory access.
> >
> > I have a very vague and possibly wrong memory that the another way
> > to get a bus error is to mmap a file into an area which is too big
> > for the file, then read/write the area beyond the end of the file.
> > Or perhaps that was on Solaris.  In any case, it might be worth
> > checking this isn't somehow related to mmap or use of mmap.
> 
> >From man mmap(2):
> 
> "SIGBUS Attempted access to a portion of the buffer that does not 
> correspond to the file (for example,
>                beyond  the end of the file, including the case where 
> another process has truncated the file)."
> 
> So this is very probable that the errors are caused by too small 
> limit of tmpfs mounted on /dev/shm.
> 
> 
> Grzegorz Kulewski
> 
> 
> 
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-- 
Darryl Dixon <esrever_otua@pythonhacker.is-a-geek.net>

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