Re: [Xen-devel] [RFC PATCH 3/4] x86/p2m : remove checks that forbid adding foreign pages between HVM guests

[Zhongze Liu <blackskygg@xxxxxxxxx>]

Hi Wei,

Yes, indeed. Before the teardown is implemented, even if I simply
remove the first
check, when I try to destroy the master domain, the domain will become a zombie,
because the refcount of the shared pages won't be correctly decreased on slave
destruction.

I don't know why this hasn't been done, so I'm separating this patch
out to serve
as a place to discuss this problem.

For this GSoC project, I would also ask if there is any temporary workaround.
While waiting for any further suggestions and answers, I'll continue
to work on the ARM side.

Cheers,

Zhongze Liu

2017-08-04 21:27 GMT+08:00 Wei Liu <wei.liu2@xxxxxxxxxx>:
> On Fri, Aug 04, 2017 at 10:20:24AM +0800, Zhongze Liu wrote:
>> Two checks in the p2m code forbids sharing physical pages among DomU's by 
>> using
>> xc_add_to_physmap_batch with XENMAPSPACE_gmfn_foreign. Just simply remove 
>> them
>> in this RFC patch to ask for suggestions on how to properly handle this.
>>
>> This is for the proposal "Allow setting up shared memory areas between VMs
>> from xl config file" (see [1]).
>>
>> [1] 
>> https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03047.html
>>
>> Signed-off-by: Zhongze Liu <blackskygg@xxxxxxxxx>
>> ---
>> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> Cc: Julien Grall <julien.grall@xxxxxxx>
>> Cc: xen-devel@xxxxxxxxxxxxx
>> ---
>>  xen/arch/x86/mm/p2m.c | 20 +++++++++++++++-----
>>  1 file changed, 15 insertions(+), 5 deletions(-)
>>
>> diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
>> index e8a57d118c..3ee4c14ed4 100644
>> --- a/xen/arch/x86/mm/p2m.c
>> +++ b/xen/arch/x86/mm/p2m.c
>> @@ -2531,8 +2531,13 @@ int p2m_add_foreign(struct domain *tdom, unsigned 
>> long fgfn,
>>       * hvm fixme: until support is added to p2m teardown code to cleanup any
>>       * foreign entries, limit this to hardware domain only.
>
> This HVM fixme needs to be fixed before the check can go away. This is
> going to be a non-trivial project, but it is going to be useful in its
> own right.

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