RE: [Xen-devel] [PATCH 6/6] Change MMU_PT_UPDATE_RESERVE_AD to support update page table for foreign domain

["Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>]

Keir Fraser wrote:
> On 01/06/2009 09:40, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:
> 
>> Thanks for suggestion. I'm always nervous on API changes.
>> 
>> I'm still considering if any other potential usage mode for patch
>> 5/6l (i.e. change page table or exchange memory for other domain),,
>> but frustratedly realize no other requirement.
> 
> The API for XENMEM_exchange already permits specifying a
> foreign domain, so
> you're just filling in the missing implementation. By the way I did
> not check your implementation, but the major subtlety is that you
> must take care
> for domains which are dying (d->is_dying). Giving new pages to
> such a domain
> is generally a bug because you race
> domain_relinquish_resources() which is
> walking the domain's page lists and freeing the pages. You
> therefore risk
> leaving a zombie domain which will not die (refcount never zero).
> 
> See for example how that is handled with page_alloc.c for calls such
> as XENMEM_populate_physmap, and how it is handled specially by
> MMUEXT_PIN_L*_TABLE. 
> 
> -- Keir

I think I missed this logic, I just make sure the domain is suspended, but 
seems be wrong because a suspended domain can still be killed and relinquish 
resources. So I will add this logic and resend the patch.

But I'm not sure if we need make sure the guest is suspended/paused during 
these functions, as exchange a page or pte in flight may cause trouble. Or we 
can leave it to user space tools, since this situation will not damage xen HV 
itself.

Thanks
Yunhong Jiang


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