Re: [Xen-devel] PVH dom0 creation fails - the system freezes

[Juergen Gross <jgross@xxxxxxxx>]

On 26/07/18 10:15, bercarug@xxxxxxxxxx wrote:
> On 07/25/2018 07:12 PM, Roger Pau Monné wrote:
>> On Wed, Jul 25, 2018 at 05:05:35PM +0300, bercarug@xxxxxxxxxx wrote:
>>> On 07/25/2018 05:02 PM, Wei Liu wrote:
>>>> On Wed, Jul 25, 2018 at 03:41:11PM +0200, Juergen Gross wrote:
>>>>> On 25/07/18 15:35, Roger Pau Monné wrote:
>>>>>>> What could be causing the available memory loss problem?
>>>>>> That seems to be Linux aggressively ballooning out memory, you go
>>>>>> from
>>>>>> 7129M total memory to 246M. Are you creating a lot of domains?
>>>>> This might be related to the tools thinking dom0 is a PV domain.
>>>> Good point.
>>>>
>>>> In that case, xenstore-ls -fp would also be useful. The output should
>>>> show the balloon target for Dom0.
>>>>
>>>> You can also try to set the autoballoon to off in /etc/xen/xl.cfg to
>>>> see
>>>> if it makes any difference.
>>>>
>>>> Wei.
>>> Also tried setting autoballooning off, but it had no effect.
>> This is a Linux/libxl issue that I'm not sure what's the best way to
>> solve. Linux has the following 'workaround' in the balloon driver:
>>
>> err = xenbus_scanf(XBT_NIL, "memory", "static-max", "%llu",
>>            &static_max);
>> if (err != 1)
>>     static_max = new_target;
>> else
>>     static_max >>= PAGE_SHIFT - 10;
>> target_diff = xen_pv_domain() ? 0
>>         : static_max - balloon_stats.target_pages;
>>
>> I suppose this is used to cope with the memory reporting mismatch
>> usually seen on HVM guests. This however interacts quite badly on a
>> PVH Dom0 that has for example:
>>
>> /local/domain/0/memory/target = "8391840"   (n0)
>> /local/domain/0/memory/static-max = "17179869180"   (n0)
>>
>> One way to solve this is to set target and static-max to the same
>> value initially, so that target_diff on Linux is 0. Another option
>> would be to force target_diff = 0 for Dom0.
>>
>> I'm attaching a patch for libxl that should solve this, could you
>> please give it a try and report back?
>>
>> I'm still unsure however about the best way to fix this, need to think
>> about it.
>>
>> Roger.
>> ---8<---
>> diff --git a/tools/libxl/libxl_mem.c b/tools/libxl/libxl_mem.c
>> index e551e09fed..2c984993d8 100644
>> --- a/tools/libxl/libxl_mem.c
>> +++ b/tools/libxl/libxl_mem.c
>> @@ -151,7 +151,9 @@ retry_transaction:
>>           *target_memkb = info.current_memkb;
>>       }
>>       if (staticmax == NULL) {
>> -        libxl__xs_printf(gc, t, max_path, "%"PRIu64, info.max_memkb);
>> +        libxl__xs_printf(gc, t, max_path, "%"PRIu64,
>> +                         libxl__domain_type(gc, 0) ==
>> LIBXL_DOMAIN_TYPE_PV ?
>> +                         info.max_memkb : info.current_memkb);
>>           *max_memkb = info.max_memkb;
>>       }
>>  
>>
> I have tried Roger's patch and it fixed the memory decrease problem. "xl
> list -l"
> 
> no longer causes any issue.
> 
> The output of "xenstore-ls -fp" shows that both target and static-max
> are now
> 
> set to the same value.

Right.

Meaning that it will be impossible to add memory to PVH dom0 e.g. after
memory hotplug.


Juergen

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