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

[Alistair Francis]

On Tue, Oct 4, 2016 at 12:56 AM, Markus Armbruster <address@hidden> wrote:
> Alistair Francis <address@hidden> writes:
>
>> On Thu, Sep 29, 2016 at 10:36 PM, Markus Armbruster <address@hidden> wrote:
>>> Alistair Francis <address@hidden> writes:
>>>
>>>> On Thu, Sep 29, 2016 at 2:24 AM, Markus Armbruster <address@hidden> wrote:
>>>>> Alistair Francis <address@hidden> writes:
>>>>>
>>>>>> Signed-off-by: Alistair Francis <address@hidden>
>>>>>> Reviewed-by: Peter Maydell <address@hidden>
>>>>>> ---
>>>>>> 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.
>>
>> I have a patch that adds known restrictions.
>>
>>>
>>>>>> +    <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.
>>
>> I disagree. It tells QEMU to treat the image as just a dumb blob,
>> instead of loading it as and ELF file. I thin force-raw makes sense as
>> the user is telling QEMU that the image should be treated as a raw
>> image, no matter what it actually is.
>
> I'm still confused then.
>
> I can see two possible features here, and based on your documentation
> and commentary, I can't tell which one you implemented:
>
> 0. QEMU can load raw files and ELF executable files.  Raw files are
> loaded verbatim at a the specified address.  ELF executable files are
> loaded by an ELF loader, which loads the program header table's loadable
> segments.
>
> 1. force-raw overrides the ELF detection, to let you load an ELF
> executable file verbatim, as if it was raw.

This is what the option does. It forces the image to be treated as a raw image.

I'm sorry if I implied it was option 2 instead.

Thanks,

Alistair

>
> 2. force-raw lets you overrides the ELF file's load address: the ELF
> loader uses the specified address instead of the address contained in
> the ELF file.
>
> Which one is it?  Your latest answer strongly suggests 1, but prior
> answers have made me suspect 2.
>