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 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.

If we were talking about large code difference (and if I am wrong about
what I said above) I'd agree that adding a few KB for x86 is OK. But
given that we are talking about a couple of lines of code ifdef'd by
'ARM' (and maybe a comment explaining why) I'd prefer not adding
anything for x86.

-boris


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