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

[Julien Grall <julien.grall@xxxxxxx>]

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?
> > 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 :).

--
Julien Grall

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