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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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.


Thanks,
Razvan

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