Re: [Xen-devel] [PATCH] xen/arm: p2m: configure pa_range_info table to support 42 bit PA systems.

[Feng Kan OS <fengkan@xxxxxxxxxxxxxxxxxxxxxx>]

On 3/18/19 4:05 AM, Julien Grall wrote:
> Hello,
> 
> On 15/03/2019 23:57, Feng Kan OS wrote:
>> On 3/15/19 4:21 AM, Julien Grall wrote:
>>> On 15/03/2019 08:34, Vishnu Pajjuri OS wrote:
>>>> Current pa_range_info table's configuration prevents 42 bit PA systems
>>>> from booting DOM0. This patch modifies t0sz=22 and root_order=3
>>>> to expose 42-bit IPA (Intermediate Physical Address).
>>>>
>>>> It is required for hardware which is having peripherals above 40 bits.
>>>
>>> The commit message is a bit confusing, you first say Xen does not boot
>>> on 42-bits PA systems but towards the end of the commit you restrict to
>>> only platform that have address wired above 40-bits.
>>>
>>> The limitation was introduced because existing platform back then had
>>> all address wired below 40-bits. So we can limit to 40-bits and allocate
>>> 2 pages rather than 8 pages for each page-tables.
>>>
>>> While I am perfectly fine to see this for Dom0, I am still unsure this
>>> is the right things for guest. Do you currently have any use case for
>>> 42-bits (4TB) guest?
>> I agree there is no use case at the moment for 42 bit guest, it is a fix
>> for DOM0 booting with peripheral and memory above 40 bits of PA. The
>> other option is to keep separate table for the guest?
> 
> Having a separate table is a possibility. Although, it would require a 
> bit more work than that. VCR_EL2, P2M_ROOT_LEVEL, P2M_ROOT_ORDER would 
> now become per-domain.
> 
> I am not entirely sure such changes is worth it yet. IIRC, in the 
> previous version of this patch, this would be fine for you to allocate 
> more memory as you have a lot of memory. Am I correct?
Yes, eMAG can go upto 1TB of RAM. We will reformat commit description.
> 
>>>
>>> I can't remember which other platforms support 42-bits PA. I think at
>>> that time it was X-Gene. As long as no current embedded platform we
>>> support use 42-bit PA, this change may be ok. Stefano do you recall what
>>> was the platform?
>> Ampere eMAG platform is essentially the continuation of X-Gene. These
>> systems are targeted as servers with upto 1TB of RAM.
> 
> So my memory hasn't failed yet :).
> 
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