Re: [PATCH v4 2/2] xen/arm: Enlarge identity map space to 10TB

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

> On 16 Oct 2023, at 17:31, Julien Grall <julien@xxxxxxx> wrote:
> 
> 
> 
> On 16/10/2023 16:07, Bertrand Marquis wrote:
>> Hi,
>>> On 16 Oct 2023, at 16:46, Julien Grall <julien@xxxxxxx> wrote:
>>> 
>>> CC Henry
>>> 
>>> Hi Alexey,
>>> 
>>> On 16/10/2023 15:24, Alexey Klimov wrote:
>>>> On Mon, 16 Oct 2023 at 15:13, Luca Fancellu <Luca.Fancellu@xxxxxxx> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>>> On 16 Oct 2023, at 15:00, Bertrand Marquis <Bertrand.Marquis@xxxxxxx> 
>>>>>> wrote:
>>>>>> 
>>>>>> Hi
>>>>>> 
>>>>>> +Luca and Rahul
>>>>>> 
>>>>>>> On 16 Oct 2023, at 15:54, Julien Grall <julien@xxxxxxx> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> On 16/10/2023 09:44, Michal Orzel wrote:
>>>>>>>> Hi,
>>>>>>> 
>>>>>>> Hi,
>>>>>>> 
>>>>>>>> On 13/10/2023 14:26, Leo Yan wrote:
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> On ADLink AVA platform (Ampere Altra SoC with 32 Arm Neoverse N1 
>>>>>>>>> cores),
>>>>>>>>> the physical memory regions are:
>>>>>>>>> 
>>>>>>>>>  DRAM memory regions:
>>>>>>>>>    Node[0] Region[0]: 0x000080000000 - 0x0000ffffffff
>>>>>>>>>    Node[0] Region[1]: 0x080000000000 - 0x08007fffffff
>>>>>>>>>    Node[0] Region[2]: 0x080100000000 - 0x0807ffffffff
>>>>>>>>> 
>>>>>>>>> The UEFI loads Xen hypervisor and DTB into the high memory, the kernel
>>>>>>>>> and ramdisk images are loaded into the low memory space:
>>>>>>>>> 
>>>>>>>>>  (XEN) MODULE[0]: 00000807f6df0000 - 00000807f6f3e000 Xen
>>>>>>>>>  (XEN) MODULE[1]: 00000807f8054000 - 00000807f8056000 Device Tree
>>>>>>>>>  (XEN) MODULE[2]: 00000000fa834000 - 00000000fc5de1d5 Ramdisk
>>>>>>>>>  (XEN) MODULE[3]: 00000000fc5df000 - 00000000ffb3f810 Kernel
>>>>>>>>> 
>>>>>>>>> In this case, the Xen binary is loaded above 8TB, which exceeds the
>>>>>>>>> maximum supported identity map space of 2TB in Xen. Consequently, the
>>>>>>>>> system fails to boot.
>>>>>>>>> 
>>>>>>>>> This patch enlarges identity map space to 10TB, allowing module 
>>>>>>>>> loading
>>>>>>>>> within the range of [0x0 .. 0x000009ff_ffff_ffff].
>>>>>>>>> 
>>>>>>>>> Fixes: 1c78d76b67 ("xen/arm64: mm: Introduce helpers to 
>>>>>>>>> prepare/enable/disable")
>>>>>>>> I don't think a fixes tag applies here given that 2TB was just a 
>>>>>>>> number we believed is enough
>>>>>>>> and all of this is platform dependent.
>>>>>>>> This can be dropped on commit if committer agrees
>>>>>>> Xen may have booted on that platform before hand. So this would be 
>>>>>>> considered a regression and therefore a tag would be warrant.
>>>>>>> 
>>>>>>> AFAICT, the commit is only present on the upcoming 4.18. So the 
>>>>>>> question is whether Xen 4.17 booted out-of-the-box on ADLink? If the 
>>>>>>> answer is yes, then we need to add a Fixes tag. But the correct one 
>>>>>>> would be
>>>>>>> 
>>>>>> 
>>>>>> @Rahul or Luca: could you give an answer here ?
>>>>>> I know you used Xen on an AVA platform but was it booting out of the box 
>>>>>> ?
>>>>> 
>>>>> I can’t say for Xen 4.17, but our nightly job has run successfully on AVA 
>>>>> for the commit 730406ab81094115d9fb5ca00ba8d53cec1279b3
>>>>> (docs/misra: add deviations.rst to document additional deviations.)
>>>>> 
>>>>> We are not applying any patch for it to run on AVA.
>>>> Most likely it is because your UEFI/BIOS firmware is 2.x, for instance
>>>> 2.04.100.07.
>>>> This fix if for AVA machine with older UEFI firmware 1.07.300.03.
>>> 
>>> OOI, why not updating your firmware? I was expecting that it would also 
>>> contain other critical fixes.
>>> 
>>> With this in mind, I am now more in two mind to ask to merge this patch in 
>>> Xen 4.18. On one hand, I understand it will help to boot on AVA machine 
>>> with an older firmware. But on the other hand this is changing the memory 
>>> layout quite late in the release. The risk seems limited because Xen is not 
>>> loaded at the top of the virtual address space (there is the directmap 
>>> afterwards).
>>> 
>>> Henry (as the release manager) and others, any opinions?
>> With the new information, I think it should be stated that it is fixing an 
>> issue on AVA platforms using an old firmware and platforms with and 
>> up-to-date firmware are not impacted.
>> It is an important information to keep in mind that this is not a fix to be 
>> backported for example (and for me the fixes tag should not be kept).
> 
> IMHO, the fixes tag should not be based solely on the AVA platform. It should 
> be based on whether this could sensibly affect others. Right now, there is 
> nothing that would indicate either way.
> 
> And therefore a Fixes tag is sensible. This doesn't mean I would want to 
> backport it right now (note that only 4.18 is affected). But this could 
> change in the future if we get another report (post-4.18) on a platform where 
> there are no other workaround.
> 
> Stefano any opinions?
> 
>> On whether or not it should go in the release, I am not quite sure that 
>> making a change in the layout at that stage is a good idea unless it is 
>> fixing a critical issue (which is not the case here).
>> So i would vote no but not go against if someone argue to have it in.
> 
> I agree with your statement with the information we have today. But it could 
> become a critical issue if another platform is hit by the same issue and have 
> no other workaround.

I am more seeing this as adding support for platforms with a topology that was 
not supported until now (because it was not encountered) rather than fixing a 
bug which is why i find it a bit odd to say that it is fixing some issue. But 
definitely this is an opinion and nothing that i could argue on :-)

Cheers
Bertrand

> 
> Cheers,
> 
> -- 
> Julien Grall