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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.10.16 at 14:53, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 10/05/2016 03:47 PM, Jan Beulich wrote:
>>>>> On 05.10.16 at 14:44, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> On 10/05/2016 03:29 PM, 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().
>>>
>>> That's an accident, in hvmemul_cmpxchg() p_old is not being used at all
>>> so IIRC I've simply latched onto p_new. All we're interested in are
>>> reads from the supplied buffer overriding the guest's actual memory.
>> 
>> So am I to understand you'll submit a patch replacing p_old with
>> p_new there (which would have the same effect as simply deleting
>> that code, except for the doc aspect)?
> 
> Yes, I'll get on that. I'll also const-ify the p_new pointer unless
> there are objections.

You won't be able to - that's where I started actually.

Jan


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