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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

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?

Thanks,
Tamas

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