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Re: [Xen-devel] 4.2 TODO update

  • To: "Olaf Hering" <olaf@xxxxxxxxx>
  • From: "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>
  • Date: Tue, 14 Feb 2012 08:47:25 -0800
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, tim@xxxxxxx, ian.campbell@xxxxxxxxxx
  • Delivery-date: Tue, 14 Feb 2012 16:48:07 +0000
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> On Tue, Feb 14, Andres Lagar-Cavilla wrote:
>> Why? Because it's really really hard to guarantee we'll go to sleep in
>> an
>> atomic context. The main use for wait queues (imho) is in hvm_copy, and
>> there's a zillion paths going into hvm_copy (copy_from/to_user!) with
>> all
>> ways of bumping the preemption count.
> If the guests pagetable is paged out this code path will trigger, then
> one of the hypercalls returns an error and the guest runs into a BUG().
> I think it was decrease_reservation, or similar.

Unlikely to be something specific about decrease_reservation. If the guest
page table is paged out, then copy_from_user for any hypercall, or,
"virtual address to gfn" for any emulation will run into this.

Now, even an innocent-looking rcu lock anywhere in this code path will
crash the host if we go into a wait queue. Hence my concern.

> Another thing reported by Huawei on this list was somewhere in the
> emulation code where a gfn_to_mfn() failed.

Can you point to the original report? Is there anything more specific?

> What other way exist to make paging 100% transparent to the guest?

Don't page out page table pages? I know you were not expecting that...


> Olaf

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