[Qemu-devel] Developing custom device for ARM

[Qemu-devel] Developing custom device for ARM

[Igor R]

Hello,

I implemented a simple sys_bus device that only communicates through
port io. The purpose of this device is to be accessible via /dev/port
only, with no need for any additional kernel modules.
I Defined the memory region using memory_region_init_io and registered
it using sysbus_add_io.
I registered the memory region in address 0x9000, and defined custom
read and write functions for it.
In the guest Linux I used dd if=/dev/port skip=$((0x9000)) count=1
bs=1 to access the port io registers.

This worked great for x86 and mips (malta) machines, however, on an
arm (versatilepb) machine, I couldn't get this method to work.
The reads and writes from 0x900X addresses did not get through to my device.

Further inspection revealed that in arm, the kernel tries to reference
the *virtual* address 0x9000, while in mips it references the virtual
address 0x8b009000.
(In x86 a different set of opcodes are used to access the port io area).

Why isn't this method working on arm?

Thanks.

Re: [Qemu-devel] Developing custom device for ARM

[Peter Maydell]

On 23 February 2016 at 15:47, Igor R <boost.lists@gmail.com> wrote:
> I implemented a simple sys_bus device that only communicates through
> port io. The purpose of this device is to be accessible via /dev/port
> only, with no need for any additional kernel modules.
> I Defined the memory region using memory_region_init_io and registered
> it using sysbus_add_io.
> I registered the memory region in address 0x9000, and defined custom
> read and write functions for it.
> In the guest Linux I used dd if=/dev/port skip=$((0x9000)) count=1
> bs=1 to access the port io registers.
>
> This worked great for x86 and mips (malta) machines, however, on an
> arm (versatilepb) machine, I couldn't get this method to work.
> The reads and writes from 0x900X addresses did not get through to my device.

> Further inspection revealed that in arm, the kernel tries to reference
> the *virtual* address 0x9000, while in mips it references the virtual
> address 0x8b009000.
> (In x86 a different set of opcodes are used to access the port io area).

/dev/port and the IO ports implemented in QEMU with sysbus_add_io
are an x86-centric idea (they correspond to what you get from
doing inb and outb instructions). What an architecture without
real IO ports decides to map /dev/port to do is a kernel level
decision. What QEMU might or might not use sysbus_add_io for
is a different decision, and there's no reason the two should
line up.

For ARM QEMU, sysbus_add_io is just not used, because it doesn't
make sense to try to add a device with IO ports to an ARM board
(with the special case exception of a PCI card, which has its
own mechanism for declaring that it has PCI IO ports).

I'm not sure what the kernel does with /dev/port but it looks like
it sets it up to do mmio accesses to the PCI IO window.

thanks
-- PMM