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Re: [PATCH qemu v8 3/3] spapr: Implement Open Firmware client interface

From: Alexey Kardashevskiy
Subject: Re: [PATCH qemu v8 3/3] spapr: Implement Open Firmware client interface
Date: Thu, 12 Mar 2020 12:26:21 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 11/03/2020 20:43, Paolo Bonzini wrote:
> On 10/03/20 06:07, Alexey Kardashevskiy wrote:
>> The PAPR platform which 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 hashmap
>> ihandle -> [phandle].
>> Before the guest started, the used memory is:
>> 0..4000 - the initial firmware
>> 10000..180000 - stack
>> This OF CI does not implement "interpret".
>> 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
>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> Thank you very much Alexey!  At least, since it can run petitboot, it's
> not completely useless.
> Acked-by: Paolo Bonzini <address@hidden>

Cool, thanks! Have you tried it yourself or you are just taking my word
for it? :)


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