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Re: Qemu and ARM secure state.

From: Peter Maydell
Subject: Re: Qemu and ARM secure state.
Date: Tue, 9 Nov 2021 10:55:47 +0000

On Mon, 8 Nov 2021 at 22:09, Jean-Christophe DUBOIS <jcd@tribudubois.net> wrote:
> OK, so one problem seems to be that PSCI-via-SMC is enabled on i.MX6UL
> when there is no built in PSCI related function on this processor.
> According the Linux DTS, i.MX7 (solo and dual) processors have a
> somewhat PSCI related "entry-method"
> (https://github.com/torvalds/linux/blob/master/arch/arm/boot/dts/imx7s.dtsi).
> But it is not clear to me how this is used and this seems a bit strange
> as "entry-method" seems to be mostly used on arm64 and there is no other
> PSCI related information in the i.MX7 DTS files.

Yeah, PSCI was an interface introduced mostly with aarch64. In the
32-bit world bringing up multiple CPUs was complete anarchy -- the
way the kernel told the secure firmware that it should start up
a secondary core was entirely determined by the firmware, and
the kernel had to have board-specific code to do this. (For the
32-bit imx boards I think this is in arch/arm/mach-imx/src.c.)
For aarch64 we had a clean slate and took the opportunity to insist
that all boards did it the same way, ie using PSCI. (There are other
useful things PSCI allows, but standardising secondary boot is the
one that matters for this discussion.) So if a platform's firmware
implements PSCI, all the dts file has to do is say so, and then
there's no need for board-specific "start secondary CPUs" code.
PSCI does define an aarch32 interface, but there are a lot of
legacy older boards (and new flavours of boards in long-standing
design families) which still do things the old way in aarch32 land.

Typically the top-level "PSCI is available" node is added by the firmware.
(QEMU will do this too when it's emulating PSCI firmware) -- if the
board code enables the psci-conduit it will add an appropriate psci
node in the hw/arm/boot.c code.)

> As a matter of fact
> previous quad or dual i.MX6 were not supporting PSCI. Instead they were
> using a proprietary method through the internal SRC device (and i.MX7
> also has a similar internal SRC device). But let's assume Linux on i.mx7
> is actually using PSCI to handle processors.
> Thinking about it, I guess this might be u-boot that sets an EL3 monitor
> software that is able to handle PSCI requests for the Linux kernel. If
> this is the case, it make sense that Qemu emulates the PSCI services
> normally provided by u-boot to be able to boot linux directly (without
> booting a real u-boot prior to linux). All  is well and nice.

Yes, that's the way it works. The EL3 firmware is supposed to provide

For aarch64 the kernel is never entered in EL3 -- it will always run at EL2
or EL1. (This is unlike aarch32, where in some cases you might run the
kernel in secure-SVC, although even there starting the kernel in
NS-SVC or NS-Hyp is more common.)

> But then if I want to boot and test the u-boot binary (or any trusted OS
> for the matter) on a Qemu emulated i.MX7 (to later boot an hypervisor or
> an OS), it would be rather strange that any PSCI related service
> requested by the hypervisor/OS  would be handled by Qemu directly and
> not by the u-boot code (or any other EL3 code) running on the processor.

Exactly. This is why the board code is supposed to set things
up so that if we are starting the guest code in EL1 or EL2
then we enable the PSCI-via-SMC support, and if we're starting
the guest code in EL3 then we do not.

> How is it supposed to work? How can I tell Qemu (dynamically?) if I want
> it to provide (or not) the PSCI services (and more generally SMC/HVC
> services).

If you want PSCI via SMC or HVC, then set the psci-conduit
If you do not want QEMU to provide PSCI, then leave psci-conduit
at its default (which is QEMU_PSCI_CONDUIT_DISABLED).

How can I tell it that I want to handle all SMC/EL3 services
> by myself even if the "psci-conduit" is already set to SMC in Qemu?

It's the imx7 code that's setting psci-conduit, so it should
not do that if it doesn't want that (and also should either
start or not start the secondary cpus powered off, depending
on what the hardware-to-firmware interface is supposed to be.)
This is a bit awkward, because the boards we initially wanted
PSCI for (notably virt) don't have an SoC object, so the code
creating CPUs is in the same source file as the code that knows
whether it's booting a kernel directly or not, and so it just
open-codes the decision logic. With the imx, the CPU creation
is in the source code for the SoC object which is abstracted
away from the board model code. So we'd need to sort out how
to plumb that information into the SoC object (or have the SoC
object's code that creates the CPUs call some function to find out).

-- PMM

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