Re: [RFC PATCH v2 0/4] arm64: realm: Support for probing RSI earlier

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, May 29, 2026 at 01:27:01PM +0100, Suzuki K Poulose wrote:
> On 28/05/2026 17:06, Catalin Marinas wrote:
> > On Tue, May 05, 2026 at 04:57:38PM +0100, Suzuki K Poulose wrote:
> > > This is an updated series, addressing the review comments from AI agent on
> > > the version 1 [0] of the series, (some of which were documented as short comings).
> > > See below for the changes.
> > > 
> > > The Realm Guest linux support is broken without rodata=full (fortunately default
> > > for arm64), as we detect the RSI support after we have created the Linear map
> > > with Block/Contiguous mappings. If the boot CPU doesn't support BBML2_NOABORT
> > > (there are CPUs out there with FEAT_RME and no - useable - BBML2_NOABORT)
> > > we are then not able to split the page tables down to PTE level if the system
> > > as such doesn't support BBML2.
> > > 
> > > See the following link for the discussion.
> > > 
> > > https://lore.kernel.org/all/20260330161705.3349825-2-ryan.roberts@arm.com/
> > > 
> > > The available options are :
> > >   1. Start with PTE level mappings at paging_init() and then "FOLD" the page tables
> > >      to Block/Cont mappings after we have the full picture available. Looking at the
> > >      future (with BBML3), this might mean "additional work" for most of the systems
> > >      at boot. But not bad as splitting them ?
> > >   2. Hold the secondary CPUs in busy loop with MMU disabled and split the mappings
> > >      by the boot CPU with MMU off (if Boot CPU can't support BBML2). This is tricky
> > >      with the page allocations required to add the page-tables.
> > >   3. Move the detection of Realm support earlier to make a better decision for
> > >      paging_init(), with an added bonus of earlycon support for Realms without
> > >      the user having to work out the "top bit" for the Realm.
> > > 
> > > This series is an attempt to implement (3) (without the earlycon support). We try
> > > to probe the PSCI conduit early from the DT/ACPI. DT is not flattened at this time.
[...]
> > Could we instead add a more informative message in arm64_rsi_init() if
> > !force_pte_mappings() && !cpu_supports_bbml2_noabort() (before
> > is_realm_world() becomes true)? Well, it may not print anything if the
> > early console is not set up yet.
> 
> That is true, but with some expertise you may be able to enable earlycon
> and may be we could get some new mechanism for "earlycon"  for Realms.
> 
> The other way to look at is:
> 
> When the system doesn't support BBML2 Abort:
> 
> Creating block/Cont mappings to start with and then splitting it to PTE
> is quite difficult as we :
> 1. Need to allocate pages for leaf level tables
> 2. Hold the other CPUs in tight loop

Agree, that's not easily possible at runtime.

> Instead, creating the block/CONT levels from a fully "page level"
> mappings are easier, as we can:
> 
> 1. Can easily fold the tables to Block mapping with reclaiming the leaf
> level pagetables.
> 
> 2. Avoid the secondary CPUs dance, as they all support BBML2_NOABORT.
> 
> This shouldn't be that bad as the opposite ?

I don't think it solves our problem. Aren't we concerned with the
rodata=off && !BBML2_NOABORT && is_realm_world() case? I don't think
your second point stands.

Currently we have:

rodata=full && BBML2_NOABORT => block mappings irrespective of realms

rodata=off && BBML2_NOABORT  => block mappings first, can be split later
				if is_realm_world()

rodata=off && !BBML2_NOABORT => block mappings first, serious problem if
				is_realm_world()

It's the last case we need to fix. Starting with page mappings does
avoid the in-realm failure but the !is_realm_world() case folding to
block mappings still requires proper BBM.

-- 
Catalin