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

[Suzuki K Poulose]

On 11/06/2026 12:14, Catalin Marinas wrote:
> 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@xxxxxxx/
> > > >
> > > > 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.

I see, the case I was missing is : !is_realm_world() and !BBML2_NO_ABORT
and we want Block mapping if rodata=off. Yes, in this case we need the 
secondaries on hold, with proper BBM on the boot CPU too. Again, it is
easier to "collapsing the tables to Block" than the reverse.

Suzuki






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