Re: [Xen-devel] [PATCH 3 of 7] Use a reserved pfn in the guest address space to store mem event rings

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> At 08:30 -0800 on 01 Mar (1330590637), Andres Lagar-Cavilla wrote:
>> > At 07:47 +0000 on 01 Mar (1330588073), Ian Campbell wrote:
>> >> > > Is there no scope for making these pages owned by the domain but
>> not
>> >> > > actually part of the P2M? We can cope with that for other types
>> of
>> >> magic
>> >> > > page, can't we?
>> >
>> > It would need a new operation to map the ring into the tool that uses
>> > it; normal map-foreign-page ops need a GFN.
>>
>> Actually, confirmed: we can call xc_domain_decrease_reservation on the
>> ring after it's mapped by the helper. Guest won't get at it. Nothing
>> breaks.
>
> But that would only work if:
>  - the helper always attaches before the guest gets to run; and

The helper will ignore whatever contents there were on the page. And if
the guest is out there poking in e820 reserved ranges, then the guest has
it coming. Note that we've narrowed the window of "vulnerability".

>  - you never need to restart the helper.

The helper can re-populate the pfn every time it starts. That's
contemplated in the current patch series. In fact it can do it
"atomically" by pausing the guest.

So, it's an improvement, but it's not water-tight fool-proof.

Andres

>
> Tim.
>



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