Xen project Mailing List

On Tue, Aug 2, 2016 at 12:19 AM, sepanta s <sapanta992@xxxxxxxxx> wrote:

On Sat, Jul 23, 2016 at 3:49 PM, sepanta s <sapanta992@xxxxxxxxx> wrote:

Hi,
Is there any sample code which I can undestand how to capture the events on the gfns which have p2m_ram_shared enabled ?
I couldn't find any ... .
I would be grateful if any help , as there is not any documents through net to use :(

Should I just set the ring_page as the pages which are shared and mark them read-only, then capture the write events?

Not sure what ring_page you are talking about, but if you mark the pages read-only with mem_access you will get notifications for events that lead to unsharing with p2m_ram_shared type pages as well.

There was a function in mem-sharing.c which is intended to announce the failed unshared pages. It is "mem_sharing_notify_enomem" .
I added "mem_sharing_notify_unshare" as a new function and call it in also XEN_DOMCTL_VM_EVENT_OP_UNSHARING and "HVM_PARAM_USHARING_RING_PFN". I also added the required codes in /xen/common/vm_event.c and /tools/libxc/xc_vm_event so as
I have added a new event for the unsharing actions of a page.
Then, I wrote a sample code line xen-access and create a ring for the pages of a domain and listen to unshared events of it.

BTW, I added a function called mem_sharing_notify_unshare to mem_sharing.c and added it to __mem_sharing_unshare_page at this part:

if ( p2m_change_type_one(d, gfn, p2m_ram_shared, p2m_ram_rw) )
{
gdprintk(XENLOG_ERR, "Could not change p2m type d %hu gfn %lx.\n",
d->domain_id, gfn);
BUG();
}else {

mem_sharing_notify_unshare(d,gfn.0);
}

IMHO this duplicates a lot of what mem_access does already, I don't think there is a need for a separate notification on another ring.

You are right, xen-access should work but I couldn't change its code and couldn't get the mem-access events.
I just added the above function to be sure that unsharing a page happens and works fine. Because I couldn't get the access requests on shared-pages of a vm.
In xen-access, Instead of setting all the pages' default access to rx , I just call xc_set_mem_access for the pages with p2m_ram_shared and assign rx as the default access but there is no requests on this ring.

So by having a vm event channel listening to unsharing event, I can see the notification in xen-access . To do so, I
have used vm_event_enable which uses HVM_PARAM_SHARING_RING_PFN .
But, when I used memshrtool to share all the pages of two vms - my vm1 and its clone vm2 .
About 900 MB of the ram is shared but there are a lot of unshared events happening.

Yes, I would say that's expected.

When I do the sharing one more time, there is not any pages unshared as the total number of shared pages stay the same.

Well, if you let the domain run for a while after sharing, then you do the sharing like that again you are likely going to crash the VM.

Seems no unsharing is done as the number of shared pages are the same.
Does any page fault triggers when a write operation is done on a shared page among two vms ?

I would guess the the VM crashed and that's why you don't see any more unsharing at that point.
Yes it was crashed as I checked it.
The scenario of sharing is I use:
I pause the origin VM and then run memshrtool on origin VM and clone VM. After sharing all the pages between these two VMs,Clone VM seems to be inaccessible. The clone seems to work as the attached photo shows, its cpu time increases and it exceeds the dom0 cpu time but when I use gvncviewer to see the GUI of the Clone VM, the mouse or keyboard don't work. (origin VM is ubunut-64-1 and clone VM is ubuntu-64-clone-1). Is there anything I have missed in sharing between two VMs?

Can someone help please ? still have problem with the machine .
is it because sharing is not based on content?

Well, the emulated device contexts probably get all messed up when you just clone over the memory of the domain and that's why they don't work. The only way I got this to work is by doing a save/restore of the origin domain before doing the memory sharing.

Tamas

Re: [Xen-devel] monitor access to pages with a specific p2m_type_t