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

[Catalin Marinas]

Hi Suzuki,

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.

Looking at the patches, I'm no longer sure it's worth justifying the
complexity. Trying to recall the previous thread - does it only matter
if BBML2_NOABORT is not supported and the kernel boots with rodata=off?
I guess we can ignore big.LITTLE configurations for now since the
deployment of CCA doesn't target mobile yet.

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.

-- 
Catalin