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Re: [Qemu-discuss] How to specify memory map for QEMU driver?

From: Peter Maydell
Subject: Re: [Qemu-discuss] How to specify memory map for QEMU driver?
Date: Wed, 1 Aug 2018 17:09:31 +0100

On 1 August 2018 at 15:09, TB <address@hidden> wrote:
> New QEMU user here, reaching out for some clarification
>on the “-dtb” and “-device” flags. How can I tell QEMU to
>map a device I specify with the “-device” flag to a
>specific memory region that I know about from the Device
>Tree Blob (DTB) file?

You can't. -device cannot be used to instantiate devices
which are memory mapped, beacuse there is no way to
specify memory address, interrupt line wiring, and so on.
It is for pluggable devices (like PCI devices, ISA bus
devices, and so on).

Memory-mapped devices are created within QEMU's sources
by the code which emulates a specific board, SoC, etc.
Implementing support for a new board implies writing a
fair amount of C code.

> For example, the DTB for a kernel I’m attempting to
>emulate specifies a bus with a set of SCU registers
>(line “compatible = "arm,cortex-a9-scu"’”) and their
>memory addresses. “qemu-system-arm -M virt -device help”
>shows “name "a9-scu", bus System” on the list of supported
>devices. So it seems that I have a match between a driver
>implemented in QEMU and a device the emulated kernel
>wants to initialize!

The list of devices is not trimmed down by board: it will
be the same regardless of what argument you pass to -M,
and doesn't imply that any particular device works with
any particular board.

The "virt" board doesn't work with the cortex-a9,
which is why it doesn't create the A9-specific SCU

Finally, you're using a slightly older QEMU, which was
a bit over-loose with what devices it listed in the
-device help option, and included ones that weren't
actually usefully usable by the user. We've tightened
this up and newer QEMU versions don't list the
cortex-a9-scu device (because it's not in fact
possible to create it on the command line).

> qemu-system-arm -M vexpress-a9 -kernel <zImage-file> -initrd <initramfs-file> 
> -append console=ttyS0,115200 -dtb <dtb-file> -cpu cortex-a9 -device a9-scu

This doesn't work (as you've noticed) and in any case
the vexpress-a9 board already creates an a9-scu for you.

> Throws the error:
> “qemu-system-arm: Option '-device a9-scu' cannot be handled by this machine”
> Switching to the “virt” machine, I get:
> “qemu-system-arm: Device a9-scu can not be dynamically instantiated”

The error messages are a bit unhelpful and we've improved
them. Current QEMU will tell you the same thing in both

qemu-system-arm: -device a9-scu: Parameter 'driver' expects pluggable
device type

> Not sure how to make sense of these errors, any advice on
> how I should be matching up DTB information (driver name,
> memory location for registers) to QEMU’s device drivers?

For QEMU, you use DTBs in one of two ways:

(1) For board models which are models of a particular
piece of real hardware, like the vexpress-a9: you
pass QEMU the DTB which is the one you'd use on
that bit of real hardware, the same way that if you
were booting a kernel on real hardware you'd pass
u-boot the correct dtb file. QEMU creates emulated
devices at the addresses where the hardware puts them,
and so they match the dtb addresses.

(2) The "virt" board model is a special case, because
it is purely-virtual hardware that QEMU controls the
specification for. In this case you don't pass QEMU
a dtb file at all -- QEMU creates one internally that
matches the device layout of the system, and passes
it to the kernel automatically.

-- PMM

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