Re: [Xen-devel] maybe revert commit c275a57f5ec3 "xen/balloon: Set balloon's initial state to number of existing RAM pages"

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/28/2017 11:30 AM, Juergen Gross wrote:
> On 28/03/17 16:27, Boris Ostrovsky wrote:
>> On 03/28/2017 04:08 AM, Jan Beulich wrote:
>>>>>> On 28.03.17 at 03:57, <boris.ostrovsky@oracle.com> wrote:
>>>> I think there is indeed a disconnect between target memory (provided by 
>>>> the toolstack) and current memory (i.e actual pages available to the guest).
>>>>
>>>> For example
>>>>
>>>> [    0.000000] BIOS-e820: [mem 0x000000000009e000-0x000000000009ffff] 
>>>> reserved
>>>> [    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] 
>>>> reserved
>>>>
>>>> are missed in target calculation. The hvmloader marks them as RESERVED 
>>>> (in build_e820_table()) but target value is not aware of this action.
>>>>
>>>> And then the same problem repeats when kernel removes 
>>>> 0x000a0000-0x000fffff chunk.
>>> But this is all in-guest behavior, i.e. nothing an entity outside the
>>> guest (tool stack or hypervisor) should need to be aware of. That
>>> said, there is still room for improvement in the tools I think:
>>> Regions which architecturally aren't RAM (namely the
>>> 0xa0000-0xfffff range) would probably better not be accounted
>>> for as RAM as far as ballooning is concerned. In the hypervisor,
>>> otoh, all memory assigned to the guest (i.e. including such backing
>>> ROMs) needs to be accounted.
>> On the Linux side we should not include in balloon calculations pages
>> reserved by trim_bios_range(), i.e. (BIOS_END-BIOS_BEGIN) + 1.
>>
>> Which leaves hvmloader's special pages (and possibly memory under
>> 0xA0000 which may get reserved). Can we pass this info to guests via
>> xenstore?
> I'd rather keep an internal difference between online pages and E820-map
> count value in the balloon driver. This should work always.

We could indeed base calculation on initial state of e820 and not count
the holes toward ballooning needs. I am not sure this will work for
memory unplug though, where a hole can be created in the map and we will
be supposed to handle disappearing memory via ballooning.

Or am I creating a problem where none exists?

-boris