Re: [PATCH v2 2/4] python/qemu: accel: Add list_accel() method

[Wainer dos Santos Moschetta <wainersm@redhat.com>]

On 12/9/19 10:52 PM, Cleber Rosa wrote:
> On Fri, Dec 06, 2019 at 04:34:31PM -0500, Wainer dos Santos Moschetta wrote:
>> Since commit cbe6d6365a48 the command `qemu -accel help` returns
>> the list of accelerators enabled in the QEMU binary. This adds
>> the list_accel() method which return that same list.
>>
>> Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
>> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>   python/qemu/accel.py | 23 +++++++++++++++++++++++
>>   1 file changed, 23 insertions(+)
>>
>> diff --git a/python/qemu/accel.py b/python/qemu/accel.py
>> index cbeac10dd1..746b7e68f5 100644
>> --- a/python/qemu/accel.py
>> +++ b/python/qemu/accel.py
>> @@ -14,7 +14,11 @@ accelerators.
>>   # the COPYING file in the top-level directory.
>>   #
>>   
>> +import logging
>>   import os
>> +import subprocess
>> +
>> +LOG = logging.getLogger(__name__)
>>   
>>   # Mapping host architecture to any additional architectures it can
>>   # support which often includes its 32 bit cousin.
>> @@ -23,6 +27,25 @@ ADDITIONAL_ARCHES = {
>>       "aarch64" : "armhf"
>>   }
>>   
>> +def list_accel(qemu_bin):
>> +    """
>> +    List accelerators enabled in the QEMU binary.
>> +
>> +    @param qemu_bin (str): path to the QEMU binary.
>> +    @raise Exception: if failed to run `qemu -accel help`
>> +    @return a list of accelerator names.
>> +    """
>> +    if not qemu_bin:
>> +        return []
>> +    try:
>> +        out = subprocess.check_output("%s -accel help" % qemu_bin, shell=True)
> There's no need to use a shell here.  This could become:
>
>     out = subprocess.check_output([qemu_bin, '-accel' 'help'])

Ack

>
>> +    except:
>> +        LOG.debug("Failed to get the list of accelerators in %s" % qemu_bin)
>> +        raise
>> +    lines = out.decode().splitlines()
> And maybe discard the first line earlier with:
>
>     lines = out.decode().splitlines()[1:]
>
> Also, you could avoid the manual decode() with the `universal_newlines`
> option to subprocess.check_output(), ie:
>
>     accels = subprocess.check_output([qemu-bin, '-accel', 'help'],
>                                      universal_newlines=True).splitlines()[1:]

Nice. v3 will have universal_newlines=True.

>
>> +    # Skip the first line which is the header.
>> +    return [l.strip() for l in lines[1:] if l]
>> +
> I think that the `if l` check can actually hide undesirable behavior
> (bugs) in the `qemu -accel ?` output.  I don't remember seeing
> `-$(option) ?` returning empty strings but doesn't mean it couldn't
> and shouldn't).
>
> I do remember `-machine ?` returning random non-printable characters
> that turned out to be a bug, though.

Double-checking: are you suggesting to remove the 'if not empty' check 
so that bugs on output could emerge?

Thanks!

- Wainer

>
>>   def kvm_available(target_arch=None):
>>       host_arch = os.uname()[4]
>>       if target_arch and target_arch != host_arch:
>> -- 
>> 2.21.0
>>
> - Cleber.