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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/20/2016 09:41 AM, Julien Grall wrote:
>
>
> On 20/07/2016 14:33, Boris Ostrovsky wrote:
>> On 07/20/2016 08:33 AM, Julien Grall wrote:
>>> Hi,
>>>
>>> On 14/07/16 14:37, Stefano Stabellini wrote:
>>>> On Wed, 13 Jul 2016, 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:
>>>>>>> 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.
>>>>
>>>> I agree with Julien. On ARM we have to increase maxmem because I don't
>>>> think that shattering a superpage is acceptable for just a few KBs. In
>>>> fact, it's not much about increasing maxmem, but it's about keeping
>>>> the
>>>> allocation of guest memory to the value passed by the user in
>>>> "memory",
>>>> so that it can be done in the most efficient way possible. (I am
>>>> assuming users are going to allocate VMs of 2048MB, rather than
>>>> 2049MB.)
>>>>
>>>> I wouldn't want to end up adding to the performance tuning page on the
>>>> wiki "Make sure to add 1 more MB to the memory of your VM to get the
>>>> most out of the system."
>>>>
>>>> I know that the location of the ACPI blob on x86 is different in guest
>>>> memory space, but it seems to me that the problem would be the
>>>> same. Do
>>>> you have 1 gigabyte pages in stage-2 on x86? If so, I would think
>>>> twice
>>>> about this. Otherwise, if you only have 4K and 2MB allocations,
>>>> then it
>>>> might not make that much of a difference.
>>>
>>> Looking at the x86 code, 1 gigabyte pages seems to be supported.
>>>
>>> Boris, do you have any opinions on this?
>>
>>
>> I don't think I understand the superpage shattering argument.  In x86
>> the tables live in regular RAM and a guest is free to use those
>> addresses as regular memory.
>>
>> This apparently is different from how ARM manages the tables (you said
>> in an earlier message that they are not part of RAM) so I can see that
>> taking memory from RAM allocation to store the tables may affect how
>> mapping is done, potentially causing GB pages to be broken.
>>
>> In fact (and I am totally speculating here) padding memory for x86 may
>> actually *cause* shattering because we will have (for example) 2049MB of
>> RAM to deal with.
>
> Correct me if I am wrong. On your series you are populating the page
> at a specific address for the ACPI tables separately to the RAM
> allocation. So you will shatter GB pages if the user provides 2048MB
> because the ACPI tables is accounted in the 2048MB.

And to be honest I am not convinced this was a well selected address
(0xfc000000). I am actually thinking about moving it down (this may
require changing dsdt.asl). I don't know whether I will actually do it
in this version but it is certainly a possibility.

-boris

>
> The plan is to call setmaxmem with the size of the RAM + size of the
> ACPI tales. The RAM will still be 2048MB and therefore GB pages may be
> used.
>



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