Re: [Xen-devel] [v7][RFC][PATCH 06/13] hvmloader/ram: check if guest memory is out of reserved device memory maps

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.11.14 at 10:40, <tiejun.chen@xxxxxxxxx> wrote:
> On 2014/11/3 16:56, Jan Beulich wrote:
>>>>> On 03.11.14 at 06:49, <tiejun.chen@xxxxxxxxx> wrote:
>>> On 2014/10/31 16:20, Jan Beulich wrote:
>>>>>>> On 31.10.14 at 07:21, <kevin.tian@xxxxxxxxx> wrote:
>>>>>>    From: Chen, Tiejun
>>>>>> Sent: Friday, October 31, 2014 1:41 PM
>>>>>> On 2014/10/30 17:20, Jan Beulich wrote:
>>>>>>> Thinking about this some more, this odd universal hole punching in
>>>>>>> the E820 is very likely to end up causing problems. Hence I think
>>>>>>> this really should be optional behavior, with pass through of devices
>>>>>>> associated with RMRRs failing if not used. (This ought to include
>>>>>>> punching holes for _just_ the devices passed through to a guest
>>>>>>> upon creation when the option is not enabled.)
>>>>>>
>>>>>> Yeah, we had a similar discussion internal to add a parameter to force
>>>>>> reserving RMRR. In this case we can't create a VM if these ranges
>>>>>> conflict with anything. So what about this idea?
>>>>>>
>>>>>
>>>>> Adding a new parameter (e.g. 'check-passthrough') looks the right
>>>>> approach. When the parameter is on, RMRR check/hole punch is
>>>>> activated at VM creation. Otherwise we just keep existing behavior.
>>>>>
>>>>> If user configures device pass-through at creation time, this parameter
>>>>> will be set by default. If user wants the VM capable of device hot-plug,
>>>>> an explicit parameter can be added in the config file to enforce RMRR
>>>>> check at creation time.
>>>>
>>>> Not exactly, I specifically described it slightly differently above. When
>>>> devices get passed through and the option is absent, holes should be
>>>> punched only for the RMRRs associated with those devices (i.e.
>>>> ideally none). Of course this means we'll need a way to associate
>>>> RMRRs with devices in the tool stack and hvmloader, i.e. the current
>>>> XENMEM_reserved_device_memory_map alone won't suffice.
>>>
>>> Yeah, current hypercall just provide RMRR entries without that
>>> associated BDF. And especially, in some cases one range may be shared by
>>> multiple devices...
>>
>> Before we decide who's going to do an eventual change we need to
>> determine what behavior we want, and whether this hypercall is
>> really the right one. Quite possibly we'd need a per-domain view
>> along with the global view, and hence rather than modifying this one
>> we may need to introduce e.g. a new domctl.
>>
> 
> If we really need to work with a hypercall, maybe we can introduce a 
> little bit to construct that to callback with multiple entries like 
> this, for instance,
> 
> RMRR entry0 have three devices, and entry1 have two devices,
> 
> [start0, nr_pages0, bdf0],
> [start0, nr_pages0, bdf1],
> [start0, nr_pages0, bdf2],
> [start1, nr_pages1, bdf3],
> [start1, nr_pages1, bdf4],
> 
> Although its cost more buffers, actually as you know this actual case is 
> really rare. So maybe this way can be feasible. Then we don't need 
> additional hypercall or xenstore.

Conceptually, as a MEMOP, it has no business reporting BDFs. And
then rather than returning the same address range more than once,
having the caller supply a handle to an array and storing all of the
SBDFs (or perhaps a single segment would suffice along with all the
BDFs) there would seem to be an approach more consistent with
what we do elsewhere.

Jan


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