Re: [Xen-devel] [PATCH v4] altp2m: Allow the hostp2m entries to be of type p2m_ram_shared

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Jun 13, 2016 at 11:20 AM, Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
> On Mon, Jun 13, 2016 at 3:28 AM, George Dunlap <george.dunlap@xxxxxxxxxx> 
> wrote:
>> On 11/06/16 18:55, Tamas K Lengyel wrote:
>>> On Thu, May 26, 2016 at 10:17 AM, Tamas K Lengyel <tamas@xxxxxxxxxxxxx> 
>>> wrote:
>>>>
>>>> On May 26, 2016 04:40, "George Dunlap" <george.dunlap@xxxxxxxxxx> wrote:
>>>>>
>>>>> On 26/05/16 04:55, Tamas K Lengyel wrote:
>>>>>> Move sharing locks above altp2m to avoid locking order violation. Allow
>>>>>> applying altp2m mem_access settings when the hostp2m entries are shared.
>>>>>> Also, do not trigger PoD for hostp2m when setting altp2m mem_access to
>>>>>> be
>>>>>> in-line with non-altp2m mem_access path. Also allow gfn remapping with
>>>>>> p2m_ram_shared type gfns in altp2m views.
>>>>>>
>>>>>> When using mem_sharing in combination with altp2m, unsharing events
>>>>>> overwrite
>>>>>> altp2m entries with that of the hostp2m (both remapped entries and
>>>>>> mem_access
>>>>>> settings). User should take precaution to not share pages where this
>>>>>> behavior
>>>>>> is undesired.
>>>>>
>>>>> I'm afraid this is not acceptable.  How could this ever be even remotely
>>>>> usable?  If this is a necessary side effect of sharing, then I think the
>>>>> original functionality, of un-sharing when setting an altp2m entry is
>>>>> the only option (and not allowing sharing to happen when an altp2m is
>>>>> present for a particular gfn).
>>>>>
>>>>> Hmm, but actually this also brings up another tricky point: In an altp2m
>>>>> you can change the mfn which backs a gfn.  This would need to be handled
>>>>> properly in the reverse map, which it doesn't look like it is at the
>>>>> moment.
>>>>>
>>>>> On the whole, I think if you're going to allow a single gfn to be
>>>>> simultaneously shared and allow an altp2m for it, you're going to need
>>>>> to do a lot more work.
>>>>>
>>>>> (Sorry for not catching a lot of this before...)
>>>>>
>>>>
>>>> Well this patch resolves the locking order violation and allows the
>>>> xen-access tool's altp2m tests to pass, so it does improve on the current
>>>> situation which is a hypervisor crash. To help with the override issue the
>>>> user can apply W mem_access permission on the shared hostp2m entries. That
>>>> way they get notification through vm_event of the event that leads to
>>>> unsharing and can then reapply the altp2m changes. So IMHO this patch is
>>>> already quite workable and while it requires more setup from the userside,
>>>> the VMM side is OK with this change.
>>>
>>> Ping. Can I get an update on what the verdict is on this patch?
>>
>> To get a proper verdict requires actually taking a deeper look and
>> thinking carefully about things :-)  Sorry for the delay -- hopefully I
>> can get to this this week.
>
> Thanks, no rush ;) Just wanted to keep the discussion on the radar.
> The only caveat with what I proposed above with using mem_access to
> get notified of unsharing is that the mem_access notification is
> currently sent before the unsharing in hvm/hvm.c. This means the user
> getting the mem_access notification has to also do the unsharing
> before reapplying the altp2m changes as well. It can be done so it
> would still work just fine, just has to keep in mind. We could also
> swap around the order of things so unsharing happens before the
> mem_access notification happens but I don't think it's necessary.
>
> Tamas

PATCH ping.

Tamas

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