Re: [PATCH 00/16] KVM: arm64: selftests: Tests for protected VMs

[Fuad Tabba]

Hi Joey,

Thanks for sharing this.

On Wed, 04 Nov 2026 14:10:45 +0000, Joey Gouly <joey.gouly@xxxxxxx> wrote:
[...]
> For example I also added a 'pre-boot' stub, which looks like your
> pvm-entry.S:
>
>         https://gitlab.arm.com/linux-arm/linux-jg/-/commit/cd3ae35d89974b02b7051a0c45cc68b4df8f9d3e
>
> However I extracted the registers needed for the stub like this:
>
>         https://gitlab.arm.com/linux-arm/linux-jg/-/commit/f559d5b61fa1a37e2d7b23704bd367e079cf8eae
>
> This is because the current Realm branch I'm working on top of rejects
> writes to those system registers (TCR_EL1, CPACR_EL1 etc). Which is why
> I had to modify these functions, along with __aarch64_vcpu_add() and
> some others.

When EL2 boots a protected guest, it takes only PC and X0 from the
host, so pvm-entry.S reads the registers from a block that X0 points
to, rather than from X0-X7 like your stub. I think a block would work
for a realm too, since your test sets X0. The trampoline also sets
VBAR_EL1 and passes X0-X7 on to the guest code, for tests that install
exception handlers or take arguments.

The host side of pKVM accepts those register writes, so pkvm.c creates
the vCPUs with aarch64_vcpu_add() as for any VM and reads the
registers back, SP_EL1 and VBAR_EL1 included.

> I'm not sure how much there is to gain by sharing overall, but good to
> bring it up early!

Thanks again for raising it. I'd keep this series as it is for now.
When you post the realm tests, would it make sense for you to move the
trampoline and its boot-args block to neutral names and boot the realm
tests through them, with your register extraction in the same series?
Patch 2 is close to your "Allow VM types for arm64", but allows only
KVM_VM_TYPE_ARM_PROTECTED, so you'd only need to add the realm type to
that check.

Cheers,
/fuad