Re: [Qemu-devel] [PATCH v12 2/2] docs: Add a generic loader explanation document

[Markus Armbruster <armbru@redhat.com>]

Alistair Francis <alistair.francis@xilinx.com> writes:

> On Thu, Sep 29, 2016 at 2:24 AM, Markus Armbruster <armbru@redhat.com> wrote:
>> Alistair Francis <alistair.francis@xilinx.com> writes:
>>
>>> Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
>>> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>>> ---
>>> V11:
>>>  - Fix corrections
>>> V10:
>>>  - Split the data loading and PC setting
>>> V9:
>>>  - Clarify the image loading options
>>> V8:
>>>  - Improve documentation
>>> V6:
>>>  - Fixup documentation
>>> V4:
>>>  - Re-write to be more comprehensive
>>>
>>>  docs/generic-loader.txt | 81 +++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 81 insertions(+)
>>>  create mode 100644 docs/generic-loader.txt
>>>
>>> diff --git a/docs/generic-loader.txt b/docs/generic-loader.txt
>>> new file mode 100644
>>> index 0000000..d1f8ce3
>>> --- /dev/null
>>> +++ b/docs/generic-loader.txt
>>> @@ -0,0 +1,81 @@
>>> +Copyright (c) 2016 Xilinx Inc.
>>> +
>>> +This work is licensed under the terms of the GNU GPL, version 2 or later.  See
>>> +the COPYING file in the top-level directory.
>>> +
>>> +
>>> +The 'loader' device allows the user to load multiple images or values into
>>> +QEMU at startup.
>>> +
>>> +Loading Data into Memory Values
>>> +---------------------
>>> +The loader device allows memory values to be set from the command line. This
>>> +can be done by following the syntax below:
>>> +
>>> +     -device loader,addr=<addr>,data=<data>,data-len=<data-len>
>>> +                   [,data-be=<data-be>][,cpu-num=<cpu-num>]
>>> +
>>> +    <addr>      - The address to store the data in.
>>> +    <data>      - The value to be written to the address. The maximum size of
>>> +                  the data is 8 bytes.
>>> +    <data-len>  - The length of the data in bytes. This argument must be
>>> +                  included if the data argument is.
>>> +    <data-be>   - Set to true if the data to be stored on the guest should be
>>> +                  written as big endian data. The default is to write little
>>> +                  endian data.
>>> +    <cpu-num>   - The number of the CPU's address space where the data should
>>> +                  be loaded. If not specified the address space of the first
>>> +                  CPU is used.
>>> +
>>> +For all values both hex and decimal values are allowed. By default the values
>>> +will be parsed as decimal. To use hex values the user should prefix the number
>>> +with a '0x'.
>>
>> Unless you bypassed QemuOpts number parsing somehow, octal works as
>> well.  In case you did bypass: don't!  Command line consistency matters.
>> Follow-up patch reverting the bypass would be required.
>>
>> Not sure we want to document QemuOpts number syntax everywhere we
>> explain how a certain feature uses the command line.  A pointer to the
>> canonical place could be better.  Anyway, not something that needs
>> fixing before we commit.
>
> I didn't bypass it, octal should work as well. I have clarified that a
> bit in the doc.

Thanks.

>>> +
>>> +An example of loading value 0x8000000e to address 0xfd1a0104 is:
>>> +    -device loader,addr=0xfd1a0104,data=0x8000000e,data-len=4
>>> +
>>> +Setting a CPU's Program Counter
>>> +---------------------
>>> +The loader device allows the CPU's PC to be set from the command line. This
>>> +can be done by following the syntax below:
>>> +
>>> +     -device loader,addr=<addr>,cpu-num=<cpu-num>
>>> +
>>> +    <addr>      - The value to use as the CPU's PC.
>>> +    <cpu-num>   - The number of the CPU whose PC should be set to the
>>> +                  specified value.
>>> +
>>> +For all values both hex and decimal values are allowed. By default the values
>>> +will be parsed as decimal. To use hex values the user should prefix the number
>>> +with a '0x'.
>>> +
>>> +An example of setting CPU 0's PC to 0x8000 is:
>>> +    -device loader,addr=0x8000,cpu-num=0
>>> +
>>> +Loading Files
>>> +---------------------
>>> +The loader device also allows files to be loaded into memory. This can be done
>>> +similarly to setting memory values. The syntax is shown below:
>>> +
>>> +    -device loader,file=<file>[,addr=<addr>][,cpu-num=<cpu-num>][,force-raw=<raw>]
>>> +
>>> +    <file>      - A file to be loaded into memory
>>> +    <addr>      - The addr in memory that the file should be loaded. This is
>>> +                  ignored if you are using an ELF (unless force-raw is true).
>>> +                  This is required if you aren't loading an ELF.
>>> +    <cpu-num>   - This specifies the CPU that should be used. This is an
>>> +                  optional argument and will cause the CPU's PC to be set to
>>> +                  where the image is stored or in the case of an ELF file to
>>> +                  the value in the header. This option should only be used
>>> +                  for the boot image.
>>> +                  This will also cause the image to be written to the specified
>>> +                  CPU's address space. If not specified, the default is CPU 0.
>>
>> Using @cpu-num both for further specifying the meaning of @addr and for
>> setting that CPU's PC is awkward.  Are you sure there will never be a
>> use case where you need to specify the CPU without also setting its PC?
>>
>> To be clear: while I feel this is a question we must discuss and
>> resolve, I don't think we need to hold the series for it.
>
> I agree that this can occur. Internally in the loader framework is a
> set_pc variable.
>
> In the future we can make this user accessible and then allow that to
> decide if the PC should be set or not.

If you can't do it right away, please document it as restriction, and
add a TODO comment to lift it.

>>> +    <force-raw> - Forces the file to be treated as a raw image. This can be
>>> +                  used to specify the load address of ELF files.
>>
>> "Specifying the load address of an ELF file" sounds like loading a
>> position-independent ELF file at a particular address.  But I guess this
>> is actually for loading a file raw even though it is recognized by QEMU
>> as ELF.
>
> This option basically does make an ELF file position-independent as
> the user can control where it is loaded.

Aha.  Then the name "force-raw" is confusing.

>>> +
>>> +For all values both hex and decimal values are allowed. By default the values
>>> +will be parsed as decimal. To use hex values the user should prefix the number
>>> +with a '0x'.
>>> +
>>> +An example of loading an ELF file which CPU0 will boot is shown below:
>>> +    -device loader,file=./images/boot.elf,cpu-num=0
>>
>> Naive question: if you want to more than one thing (where "thing" is one
>> of the three cases described above), do you need a separate -device for
>> each, or can you combine them into one?
>
> You can't really squash them together. If you wanted to set two
> registers, you would need two commands.

That's okay.  It just isn't quite obvious to me in the text.

>
> Thanks,
>
> Alistair
>
>>
>>
>> Again, while my questions may lead to improvements, they can be applied
>> on top.
>>