Re: [Xen-devel] [PATCH v2 16/17] libxc/xc_dom_arm: Copy ACPI tables to guest space

[Shannon Zhao <zhaoshenglong@xxxxxxxxxx>]

On 2016/7/14 19:15, Wei Liu wrote:
> On Wed, Jul 13, 2016 at 01:08:57PM -0400, Boris Ostrovsky wrote:
>> On 07/13/2016 11:22 AM, Julien Grall wrote:
>>> Hello,
>>>
>>> On 12/07/2016 17:58, Boris Ostrovsky wrote:
>>>> On 07/12/2016 12:10 PM, Julien Grall wrote:
>>>>> On 12/07/2016 16:08, Boris Ostrovsky wrote:
>>>>>> On 07/12/2016 10:57 AM, Shannon Zhao wrote:
>>>>>>> On 2016年07月12日 22:50, Wei Liu wrote:
>>>>>>>> On Tue, Jul 12, 2016 at 10:42:07PM +0800, Shannon Zhao wrote:
>>>>>>>>>>>>>>>>>>>>>>>>>>>> Does it mean we would need to update the slack
>>>>>>>>>>>>>>>>>>>>>>>>>>>> to take into account the ACPI
>>>>>>>>>>>>>>>>>>>>>>>>>>>> blob?
>>>>>>>>>>>>>>>>>>>> Yes, we need to take into account the ACPI blob.
>>>>>>>>>>>>>>>>>>>> Probably not in the
>>>>>>>>>>>>>>>>>>>> slack but directly in mam_memkb.
>>>>>>>>>>>>>>>> Sorry, I'm not sure understand this. I found the
>>>>>>>>>>>>>>>> b_info->max_memkb but
>>>>>>>>>>>>>>>> didn't find the slack you said. And how to fix this? Update
>>>>>>>>>>>>>>>> b_info->max_memkb or the slack?
>>>>>>>>>>>> Can you calculate the size of your payload and add that to
>>>>>>>>>>>> max_memkb?
>>>>>>>>>>>>
>>>>>>>>>> Yeah, but the size will be changed if we change the tables in the
>>>>>>>>>> future
>>>>>>>>>> and this also should consider x86, right?
>>>>>>>> That could easily be solved by introducing a function to
>>>>>>>> calculate the
>>>>>>>> size, right?
>>>>>>> Oh, I'm not familiar with this. Let's clarify on this. It can add the
>>>>>>> size to max_memkb after generating the ACPI tables and before loading
>>>>>>> the tables to guest space and it doesn't have to add the size at
>>>>>>> libxl__domain_build_info_setdefault(), right?
>>>>>>
>>>>>> This was discussed before: ACPI tables are part of RAM whose size is
>>>>>> specified by the config file (and is reflected in max_memkb I
>>>>>> believe).
>>>>>> It may not be presented to the guest as RAM (i.e. on x86 it is labeled
>>>>>> by BIOS (or whoever) as a dedicated type in e820) but it still resides
>>>>>> in DIMMs.
>>>>>
>>>>> I don't think this was the conclusion of the thread. IHMO, "maxmem" is
>>>>> the amount of RAM a guest could effectively use.
>>>>>
>>>>> Whilst the ACPI tables will be in the DIMM from the host point of
>>>>> view. From a guest point of view it will be a ROM.
>>>>
>>>> The config file specifies resources provided by the host. How the guest
>>>> views those resources is not important, I think.
>>>
>>> This would need to be clarified. For instance special pages (Xenstore,
>>> Console...) are RAM from the host point of view but not taken into
>>> account in the "maxmem" provided by the user. For my understanding,
>>> some kB of the slack is used for that.
>>
>>
>> Are these pages part of guest's address space?
>>
>>
>>>
>>>>
>>>>>
>>>>> It will affect some others part of the guest if we don't increment the
>>>>> "maxmem" requested by the user. For ARM the ACPI blob will be exposed
>>>>> at a specific address that is outside of the guest RAM (see the guest
>>>>> memory layout in public/arch-arm.h).
>>>>>
>>>>> We chose this solution over putting in the RAM because the ACPI tables
>>>>> are not easily relocatable (compare to the device tree, initrd and
>>>>> kernel) so we could not take advantage of superpage in both stage-2
>>>>> (hypervisor) and stage-1 (kernel) page table.
>>>>
>>>> Maybe this is something ARM-specific then. For x86 we will want to keep
>>>> maxmem unchanged.
>>>
>>> I don't think what I described in my previous mail is ARM-specific.
>>> The pressure will be more important on the TLBs, if Xen does not use
>>> superpage in the stage 2 page tables (i.e EPT for x86) no matter the
>>> architecture.
>>>
>>> IHMO, this seems to be a bigger drawback compare to add few more
>>> kilobytes to maxmem in the toolstack for the ACPI blob. You will loose
>>> them when creating the intermediate page table in any case.
>>
>>
>> Why not have the guest ask for more memory in the config file then?
>>
>> (OK, I can see that they can't ask for a few KB since we have MB
>> resolution but they can ask for an extra 1MB)
>>
> 
> It would be trivial to have another option in xl.cfg to allow MB
> granularity. But I don't think that's a good idea. Asking for more
> memory when you don't really know how much is enough is not very useful.
> If an admin can know how much is needed, surely the library can be
> taught to obtain that knowledge, too.
> 
> We need to decide which model we should go with. And, if we decide to
> diverge, document the difference between x86 and ARM model.
> 
Hi Wei,

Do you decide how to add the size of ACPI blob to max_memkb?

Thanks,
-- 
Shannon


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