Re: [PATCH qemu v22] spapr: Implement Open Firmware client interface

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 6/28/21 02:38, BALATON Zoltan wrote:
> On Fri, 25 Jun 2021, Alexey Kardashevskiy wrote:
>> The PAPR platform describes an OS environment that's presented by
>> a combination of a hypervisor and firmware. The features it specifies
>> require collaboration between the firmware and the hypervisor.
>>
>> Since the beginning, the runtime component of the firmware (RTAS) has
>> been implemented as a 20 byte shim which simply forwards it to
>> a hypercall implemented in qemu. The boot time firmware component is
>> SLOF - but a build that's specific to qemu, and has always needed to be
>> updated in sync with it. Even though we've managed to limit the amount
>> of runtime communication we need between qemu and SLOF, there's some,
>> and it has become increasingly awkward to handle as we've implemented
>> new features.
>>
>> This implements a boot time OF client interface (CI) which is
>> enabled by a new "x-vof" pseries machine option (stands for "Virtual Open
>> Firmware). When enabled, QEMU implements the custom H_OF_CLIENT hcall
>> which implements Open Firmware Client Interface (OF CI). This allows
>> using a smaller stateless firmware which does not have to manage
>> the device tree.
>>
>> The new "vof.bin" firmware image is included with source code under
>> pc-bios/. It also includes RTAS blob.
>>
>> This implements a handful of CI methods just to get -kernel/-initrd
>> working. In particular, this implements the device tree fetching and
>> simple memory allocator - "claim" (an OF CI memory allocator) and updates
>> "/memory@0/available" to report the client about available memory.
>>
>> This implements changing some device tree properties which we know how
>> to deal with, the rest is ignored. To allow changes, this skips
>> fdt_pack() when x-vof=on as not packing the blob leaves some room for
>> appending.
>>
>> In absence of SLOF, this assigns phandles to device tree nodes to make
>> device tree traversing work.
>>
>> When x-vof=on, this adds "/chosen" every time QEMU (re)builds a tree.
>>
>> This adds basic instances support which are managed by a hash map
>> ihandle -> [phandle].
>>
>> Before the guest started, the used memory is:
>> 0..e60 - the initial firmware
>> 8000..10000 - stack
>> 400000.. - kernel
>> 3ea0000.. - initramdisk
>>
>> This OF CI does not implement "interpret".
>>
>> Unlike SLOF, this does not format uninitialized nvram. Instead, this
>> includes a disk image with pre-formatted nvram.
>>
>> With this basic support, this can only boot into kernel directly.
>> However this is just enough for the petitboot kernel and initradmdisk to
>> boot from any possible source. Note this requires reasonably recent guest
>> kernel with:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=df5be5be8735 
>>
>>
>> The immediate benefit is much faster booting time which especially
>> crucial with fully emulated early CPU bring up environments. Also this
>> may come handy when/if GRUB-in-the-userspace sees light of the day.
>>
>> This separates VOF and sPAPR in a hope that VOF bits may be reused by
>> other POWERPC boards which do not support pSeries.
>>
>> This assumes potential support for booting from QEMU backends
>> such as blockdev or netdev without devices/drivers used.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> 
> Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>


Thanks a lot for thorough reviews, much appreciated!


-- 
Alexey