Re: [PATCH v2 2/6] accel: accel_available() function

[Claudio Fontana]

On 11/26/20 2:36 PM, Eduardo Habkost wrote:
> On Thu, Nov 26, 2020 at 10:14:31AM +0100, Claudio Fontana wrote:
>> Hi Eduardo,
>>
>> On 11/25/20 9:56 PM, Eduardo Habkost wrote:
>>> This function will be used to replace the xen_available() and
>>> kvm_available() functions from arch_init.c.
>>>
>>> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
>>> ---
>>> Cc: Richard Henderson <richard.henderson@linaro.org>
>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>> Cc: Claudio Fontana <cfontana@suse.de>
>>> Cc: Roman Bolshakov <r.bolshakov@yadro.com>
>>> ---
>>>  include/sysemu/accel.h | 1 +
>>>  accel/accel.c          | 5 +++++
>>>  2 files changed, 6 insertions(+)
>>>
>>> diff --git a/include/sysemu/accel.h b/include/sysemu/accel.h
>>> index e08b8ab8fa..a4a00c75c8 100644
>>> --- a/include/sysemu/accel.h
>>> +++ b/include/sysemu/accel.h
>>> @@ -67,6 +67,7 @@ typedef struct AccelClass {
>>>      OBJECT_GET_CLASS(AccelClass, (obj), TYPE_ACCEL)
>>>  
>>>  AccelClass *accel_find(const char *opt_name);
>>> +bool accel_available(const char *name);
>>>  int accel_init_machine(AccelState *accel, MachineState *ms);
>>>  
>>>  /* Called just before os_setup_post (ie just before drop OS privs) */
>>> diff --git a/accel/accel.c b/accel/accel.c
>>> index cb555e3b06..4a64a2b38a 100644
>>> --- a/accel/accel.c
>>> +++ b/accel/accel.c
>>> @@ -46,6 +46,11 @@ AccelClass *accel_find(const char *opt_name)
>>>      return ac;
>>>  }
>>>  
>>> +bool accel_available(const char *name)
>>> +{
>>> +    return accel_find(name) != NULL;
>>
>>
>> accel_find() in its implementation allocates and then frees memory to 
>> generate the string,
>> the user of accel_available() might be unaware and overuse leading to 
>> fragmentation/performance issues?
> 
> Is that a real issue?  We had only 3 users of kvm_available() and
> xen_available() since those functions were added 10 years ago.
> 
> Do you have any suggestions on what we should do?
> 

One option I see is simply to document the behavior where accel_available() is 
declared in accel.h (ie do not use in fast path), as well as in accel_find() 
actually,
so that both accel_find() and accel_available() are avoided in fast path and 
avoid being called frequently at runtime.

Another option could be to remove the allocation completely, and use for 
example accel_find(ACCEL_CLASS_NAME("tcg")),
or another option again would be to remove the allocation and use either a 
fixed buffer + snprintf, or alloca -like builtin code to use the stack, ...

Not a big deal, but with a general utility and short name like 
accel_available(name) it might be tempting to use this more in the future?

Ciao,

Claudio