Re: [Xen-devel] Inconsistent use of set_context_data()?

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/10/16 13:29, Jan Beulich wrote:
>>>> On 05.10.16 at 14:22, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> what's the point of this being used by hvmemul_read() and
>>> hvmemul_cmpxchg(), but (namely but not limited to) not by
>>> hvmemul_write()?
>> To do introspection work, we sometimes need to modify the guest memory,
>> and there are cases, namely during hibernate / resume of Windows guests,
>> when we need to serve the "old" version of that memory to the current
>> instruction reading from it for the process to work reliably.
>>
>> The design choice here has been that the introspection application is
>> smart enough to handle writes (after all, it is the one managing the
>> buffer sent via vm_event reply), so it is intended behaviour.
> Well - the confusing thing is that for cmpxchg it's the value to be
> written which gets altered, not the value to be compared against,
> i.e. it acts as if set_context_data() was also intended to be
> present in hvmemul_write().

Can I highly suggest that writing an Introspection feature doc,
explaining some bits and pieces like this might be a very good idea?

Also, commiseration to whichever poor sole had to debug that issue...

~Andrew

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