Xen devel - Sep 2012 - Re: [PATCH] Xen backend support for paged out grant targets V4.

On Sep 21, 2012, at 2:52 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Sep 17, 2012 at 05:29:24AM -0400, Andres Lagar-Cavilla wrote:
>> On Sep 17, 2012, at 4:17 AM, Ian Campbell wrote:
>> 
>>> (I think I forgot to hit send on this on Friday, sorry. Also
>>> s/xen.lists.org/lists.xen.org in the CC line…)
>> I''m on a roll here…
>> 
>>> 
>>> On Fri, 2012-09-14 at 15:26 +0100, Andres Lagar-Cavilla wrote:
>>>> Since Xen-4.2, hvm domains may have portions of their memory
paged out. When a
>>>> foreign domain (such as dom0) attempts to map these frames, the
map will
>>>> initially fail. The hypervisor returns a suitable errno, and
kicks an
>>>> asynchronous page-in operation carried out by a helper. The
foreign domain is
>>>> expected to retry the mapping operation until it eventually
succeeds. The
>>>> foreign domain is not put to sleep because itself could be the
one running the
>>>> pager assist (typical scenario for dom0).
>>>> 
>>>> This patch adds support for this mechanism for backend drivers
using grant
>>>> mapping and copying operations. Specifically, this covers the
blkback and
>>>> gntdev drivers (which map foregin grants), and the netback
driver (which copies
>>> 
>>>                          foreign
>>> 
>>>> foreign grants).
>>>> 
>>>> * Add a retry method for grants that fail with GNTST_eagain
(i.e. because the
>>>> target foregin frame is paged out).
>>> 
>>>         foreign
>>> 
>>>> * Insert hooks with appropriate wrappers in the aforementioned
drivers.
>>>> 
>>>> The retry loop is only invoked if the grant operation status is
GNTST_eagain.
>>>> It guarantees to leave a new status code different from
GNTST_eagain. Any other
>>>> status code results in identical code execution as before.
>>>> 
>>>> The retry loop performs 256 attempts with increasing time
intervals through a
>>>> 32 second period. It uses msleep to yield while waiting for the
next retry.
>>> [...]
>>>> Signed-off-by: Andres Lagar-Cavilla
<andres@lagarcavilla.org>
>>> 
>>> Acked-by: Ian Campbell <ian.campbell@citrix.com>
>>> 
>>> Since this is more about grant tables than netback this should
probably
>>> go via Konrad rather than Dave, is that OK with you Dave?
>> 
>> If that is the case hopefully Konrad can deal with the two typos?
Otherwise happy to re-spin the patch.
> 
> So with this patch when I launch an PVHVM guest on Xen 4.1 I get this
> in the initial domain and the guest is crashed:
> 
> [  261.927218] privcmd_fault: vma=ffff88002a31dce8
7f4edc095000-7f4edc195000, pgoff=c8, uv=00007f4edc15d000
With this patch? Or with the mmapbatch v2? This is a page fault in a
foreign-mapped VMA. Not touched by this grant backend patch we are talking
about.

Does the hypervisor dump anything to its console?

At which point during xc_hvm_build do you see this? (or elsewhere in the
toolstack?)

Thanks
Andres
> 
> guest config:
>> more /mnt/lab/latest/hvm.xm 
> kernel = "/usr/lib/xen/boot/hvmloader"
> builder=''hvm''
> memory=1024
> #maxmem=1024
> maxvcpus = 2
> serial=''pty''
> vcpus = 2
> disk = [
''file:/mnt/lab/latest/root_image.iso,hdc:cdrom,r'']
> boot="dn"
> #vif = [ ''type=ioemu,model=e1000,mac=00:0F:4B:00:00:71,
bridge=switch'' ]
> vif = [ ''type=netfront, bridge=switch'' ]
> #vfb = [ ''vnc=1, vnclisten=0.0.0.0 ,vncunused=1'']
> vnc=1
> vnclisten="0.0.0.0"
> usb=1
> xen_platform_pci=1
> 
>