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

>>> 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.


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