[Xen-users] QEMU backend initialization failure.

[Paulina Szubarczyk <paulinaszubarczyk@xxxxxxxxx>]

Hi,

I encountered today a problem with starting the qemu device model during
creation of a PVHVM domain.

qemu-log:
xen be core: xen be core: xen be: watching backend path 
(backend/console/21) failed
xen be: watching backend path (backend/console/21) failed
xen be core: xen be core: xen be: watching backend path 
(backend/vkbd/21) failed
xen be: watching backend path (backend/vkbd/21) failed
xen be core: xen be core: xen be: watching backend path 
(backend/qdisk/21) failed
xen be: watching backend path (backend/qdisk/21) failed

my current system configuration:
release                : 4.6.2-040602-generic
xen_version            : 4.8-unstable
qemu                   : 2.6.50

I looked for the cause of it and on my machine the problem comes from 
creating a new posix thread. More specifically the setting of the stack 
size for a thread causes a failure of creation of the thread with the 
value EINVAL - Invalid argument.
In the tools/xenstore/xs.c:xs_watch the function 
pthread_attr_setstacksize(&attr, DEFAULT_THREAD_STACKSIZE) is called.
The DEFAULT_THREAD_STACKSIZE cannot be less then PTHREAD_STACK_MIN that 
resolves to 16 KB and this is also the default value set by the 
toolstack. But setting the stack to that value causes the EINVAL. A 
default value for the stack is 8MB. I played with the parameter 
DEFAULT_THREAD_STACKSIZE and the problem does not exist for the value 
above or equal 20KB in my case.
I saw that there was a similar raport on the list recently.
https://lists.xenproject.org/archives/html/xen-users/2016-07/msg00021.html

Paulina






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