No graphics with xen pv and Fedora qemu

No graphics with xen pv and Fedora qemu

[Michael Young]

I have been trying to trace a problem when using Fedora's qemu with a pv 
guest which is that no graphics are available. I get the errors

xen be core: xen be: watching backend path (backend/console/2) failed
xen be core: xen be: watching backend path (backend/vkbd/2) failed
xen be core: xen be: watching backend path (backend/vfb/2) failed
xen be core: xen be: watching backend path (backend/qdisk/2) failed
xen be core: xen be: watching backend path (backend/qnic/2) failed

in the qemu log file in /var/log/xen . So far I have traced it to rbd 
support in qemu, because qemu-system-i386 built with the --disable-rbd 
does have working graphics.
Does anyone have suggestions on what the problem might be, or how I might 
debug the issue further.

 	Michael Young

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Re: No graphics with xen pv and Fedora qemu

[Michael Young]

On Wed, 29 Jun 2016, Michael Young wrote:

> I have been trying to trace a problem when using Fedora's qemu with a pv 
> guest which is that no graphics are available. I get the errors
>
> xen be core: xen be: watching backend path (backend/console/2) failed
> xen be core: xen be: watching backend path (backend/vkbd/2) failed
> xen be core: xen be: watching backend path (backend/vfb/2) failed
> xen be core: xen be: watching backend path (backend/qdisk/2) failed
> xen be core: xen be: watching backend path (backend/qnic/2) failed
>
> in the qemu log file in /var/log/xen . So far I have traced it to rbd 
> support in qemu, because qemu-system-i386 built with the --disable-rbd 
> does have working graphics.

I tracked this down eventually. The failure is in tools/xenstore/xs.c in 
xs_watch when it tries to create a read pthread, because the initial stack 
size of 16384 is apparently too small with qemu with rbd enabled. I did 
get it to work if I increased the stack size to 24576.

 	Michael Young

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Re: No graphics with xen pv and Fedora qemu

[Andrew Cooper]

On 10/07/16 16:44, Michael Young wrote:
> On Wed, 29 Jun 2016, Michael Young wrote:
>
>> I have been trying to trace a problem when using Fedora's qemu with a
>> pv guest which is that no graphics are available. I get the errors
>>
>> xen be core: xen be: watching backend path (backend/console/2) failed
>> xen be core: xen be: watching backend path (backend/vkbd/2) failed
>> xen be core: xen be: watching backend path (backend/vfb/2) failed
>> xen be core: xen be: watching backend path (backend/qdisk/2) failed
>> xen be core: xen be: watching backend path (backend/qnic/2) failed
>>
>> in the qemu log file in /var/log/xen . So far I have traced it to rbd
>> support in qemu, because qemu-system-i386 built with the
>> --disable-rbd does have working graphics.
>
> I tracked this down eventually. The failure is in tools/xenstore/xs.c
> in xs_watch when it tries to create a read pthread, because the
> initial stack size of 16384 is apparently too small with qemu with rbd
> enabled. I did get it to work if I increased the stack size to 24576.

This is rather curious (and confusing).

I presume this is the xenstore reader thread in qemu which is causing
problems?

The stack size of the reader thread shouldn't need to be big at all, and
I can't see why rbd specifically would cause an issue.  Is there a stack
overflow happening?

~Andrew

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Re: No graphics with xen pv and Fedora qemu

[Andrew Cooper]

On 13/07/16 16:44, Andrew Cooper wrote:
> On 10/07/16 16:44, Michael Young wrote:
>> On Wed, 29 Jun 2016, Michael Young wrote:
>>
>>> I have been trying to trace a problem when using Fedora's qemu with a
>>> pv guest which is that no graphics are available. I get the errors
>>>
>>> xen be core: xen be: watching backend path (backend/console/2) failed
>>> xen be core: xen be: watching backend path (backend/vkbd/2) failed
>>> xen be core: xen be: watching backend path (backend/vfb/2) failed
>>> xen be core: xen be: watching backend path (backend/qdisk/2) failed
>>> xen be core: xen be: watching backend path (backend/qnic/2) failed
>>>
>>> in the qemu log file in /var/log/xen . So far I have traced it to rbd
>>> support in qemu, because qemu-system-i386 built with the
>>> --disable-rbd does have working graphics.
>> I tracked this down eventually. The failure is in tools/xenstore/xs.c
>> in xs_watch when it tries to create a read pthread, because the
>> initial stack size of 16384 is apparently too small with qemu with rbd
>> enabled. I did get it to work if I increased the stack size to 24576.
> This is rather curious (and confusing).
>
> I presume this is the xenstore reader thread in qemu which is causing
> problems?
>
> The stack size of the reader thread shouldn't need to be big at all, and
> I can't see why rbd specifically would cause an issue.  Is there a stack
> overflow happening?

Juergun has found what might be the underlying issue here.  Being
Fedora, I guess you are on a very recent qemu.

Could you try the patch on
https://lists.xenproject.org/archives/html/xen-devel/2016-07/msg01745.html
and see whether it resolves your issue?

~Andrew

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