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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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.

Peace,
 -George



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