Re: [Xen-devel] [PATCH 2/2] memory: don't hand MFN info to translated guests

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Mon, Jun 19, 2017 at 3:11 AM, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> On 19/06/17 09:15, Jan Beulich wrote:
>>>>> On 18.06.17 at 21:19, <tamas.k.lengyel@xxxxxxxxx> wrote:
>>> On Tue, Apr 4, 2017 at 1:04 PM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> wrote:
>>>> On 04/04/17 14:14, Jan Beulich wrote:
>>>>> We shouldn't hand MFN info back from increase-reservation for
>>>>> translated domains, just like we don't for populate-physmap and
>>>>> memory-exchange. For full symmetry also check for a NULL guest handle
>>>>> in populate_physmap() (but note this makes no sense in
>>>>> memory_exchange(), as there the array is also an input).
>>>>>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>
>>>> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>
>>> Unfortunately I just had time to do testing with this change and I
>>> have to report that introduces a critical regression for my tools.
>>> With this change in-place performing increase_reservation on a target
>>> domain no longer reports the guest frame number for external tools,
>>> thus completely breaking advanced use-cases that require this
>>> information to be able to do altp2m gfn remapping. This is a critical
>>> step in being able to introduce shadow-pages that are used to hide
>>> breakpoints and other memory modifications from the guest.
>>
>> While I can see your point, I'm afraid that's not how the
>> interface was meant to be used.
>
> Well the first question to ask is, is that hypercall part of the stable
> interface?  If so, then the standard should be, "Don't break people who
> call it unless there is really no other way around it."  Sure, it was a
> mistake whoever introduced that, but if Tamas is building on a "stable"
> interface he should be able to rely on that interface being maintained,
> at least until we can find a suitable replacement.
>
>  -George
>

Of course if a suitable replacement can be made that gets me the
information I need that would work too. At the moment I'm not aware of
any other hypercall I could use for this purpose.

Tamas

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