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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/10/16 14:12, Razvan Cojocaru wrote:
> On 10/05/2016 03:40 PM, Andrew Cooper wrote:
>> 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?
> Fair point, I suppose what we'd need to figure out at this point is A)
> where would the best place for this information to appear be (I assume
> somewhere on the project Wiki)

docs/features/  in tree please.

See the migration and levelling docs as examples, and the template for
suggested layout.

> , and B) what information is interesting,
> or at least complex, enough to warrant higher level descriptions?

In this case, I would suggested suggest that the Technical Details
section include some details as to what an introspection agent needs to
be able to do, using the interface, (and preferably why).

Something like "override the values from memory on an emulated read"
would certainly be helpful when referring to the emulation code.

~Andrew

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