[Xen-devel] Re: event channel in xenpaging

[Olaf Hering <olaf@xxxxxxxxx>]

On Wed, Sep 28, zhen shi wrote:

> Hi,Olaf,
> 
> I have some questions about event channel in Xenpaging to ask you.
> 1) In xenpaging it uses Inter-Domainain Commnication (IDC) between
> dom0 and domU to build bidirectional connection,but I found there is
> only an event channel notification from xen to dom0 when page faults
> happens.It seems that xenpaging_resume_page()->xc_evtchn_notify()
> doesn't make any difference.So why don't use vIRQ between dom0 and xen
> instead of IDC between dom0 and domU?

I talked with Adin Scannel about that, he has changes to use an event
channel instead of domctrl for communication. The current event channel
usage in xenpaging is a noop.

> 2)In your latest patch,[PATCH 9 of 9] xenpaging: watch the domains
> /xenpaging/num_pages xenstore value.I found some problems.
>   aãIn main(),you put the page_out process into while(1) and make the
> following change.
>      if ( interrupted )
>     victims[i].gfn = INVALID_MFN;
>     - else
>     - evict_victim(paging, &victims[i], fd, i);
> I think the" if ( interrupted )" should remove here.Because once
> handling page_in requests,the related victims slot should clear,
> then in evict_pages we can populate new page in this slot because in
> evict_page() the condition as followings:
>  /* Slot is allocated */
> + if ( victims[slot].gfn != INVALID_MFN )
> + continue;

Thanks for spotting this, that part is not correct and the comment is
now bogus as well.

> bãIn xenpaging_init (),when it goes to err,you add
> "free(dom_path);free(watch_targetpages)" in PATCH 9,but I think we
> should firstly judge if dom_path and watch_targetpages variables are
> NULL.what's more ,if it initializes paging successfullyïbefore "return
> paging",we should free (dom_path),and free(watch_targetpages) in
> xenpaging_teardown() lastly.
> cãwhen initializing xenpaging fails,and goes to err,it returns NULL to
> main() then exit main().
> If it has already opened event channel,bind event notification,opened
> connections to xen successfully when goes to err in
> xenpaging_init()ïhow about dealing with these resources such as
> xenpaging_teardown.

watch_targetpages is used in xenpaging_wait_for_event_or_timeout(), and
dom_path is used elsewhere in upcoming changes.
You are right about xenpaging_teardown(), perhaps the error path in
xenpaging_init() could make use of that function.

> 3)We have tested on Win7-32bit about 40vms to start xenpaging at the
> same time.The vm is 1G 2VCPUS,and we page out 180M.Then we put 80-90%
> memory pressure on each vm.About one hour later,there happens many
> BSOD on vms.and most blue screen code is 0x... 7F or 0x...19. We guess
> some special pages which systems need use to run may be paged out and
> cause BSOD.and sometimes print out mmio information in messages.Do you
> have any ideas about the BSOD on win7-32 bit vms?

That sounds like incorrect handling of paged-out pages in Xen, many code
paths are supposed to retry the gfn_to_mfn* calls currently, but some do
not. 
I will start to work on that bug soon.
Do these guests use the PV drivers?

Olaf

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