Re: [PATCH] via-ide: Avoid expensive operations in irq handler

[BALATON Zoltan <balaton@eik.bme.hu>]

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On Wed, 20 Oct 2021, Eduardo Habkost wrote:
> On Mon, Oct 18, 2021 at 12:10:04PM +0200, Philippe Mathieu-Daudé wrote:
>> On 10/18/21 11:51, BALATON Zoltan wrote:
>>> On Mon, 18 Oct 2021, Philippe Mathieu-Daudé wrote:
>>>> On 10/18/21 03:36, BALATON Zoltan wrote:
>>>>> Cache the pointer to PCI function 0 (ISA bridge, that this IDE device
>>>>> has to use for IRQs) in the PCIIDEState and pass that as the opaque
>>>>> data for the interrupt handler to eliminate both the need to look up
>>>>> function 0 at every interrupt and also a QOM type cast of the opaque
>>>>> pointer as that's also expensive (mainly due to qom-debug being
>>>>> enabled by default).
>>>>>
>>>>> Suggested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>>>> Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
>>>>> ---
>>>>>  hw/ide/via.c         | 11 ++++++-----
>>>>>  include/hw/ide/pci.h |  1 +
>>>>>  2 files changed, 7 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/hw/ide/via.c b/hw/ide/via.c
>>>>> index 82def819c4..30566bc409 100644
>>>>> --- a/hw/ide/via.c
>>>>> +++ b/hw/ide/via.c
>>>>> @@ -104,15 +104,15 @@ static void bmdma_setup_bar(PCIIDEState *d)
>>>>>
>>>>>  static void via_ide_set_irq(void *opaque, int n, int level)
>>>>>  {
>>>>> -    PCIDevice *d = PCI_DEVICE(opaque);
>>>>> +    PCIIDEState *d = opaque;
>>>>>
>>>>>      if (level) {
>>>>> -        d->config[0x70 + n * 8] |= 0x80;
>>>>> +        d->parent_obj.config[0x70 + n * 8] |= 0x80;
>>>>>      } else {
>>>>> -        d->config[0x70 + n * 8] &= ~0x80;
>>>>> +        d->parent_obj.config[0x70 + n * 8] &= ~0x80;
>>>>>      }
>>>>
>>>> Better not to access parent_obj, so I'd rather keep the previous
>>>> code as it. The rest is OK, thanks. If you don't want to respin
>>>> I can fix and take via mips-next.
>>>
>>> Why not? If it's OK to access other fields why not parent_obj? That
>>> avoids the QOM cast PCI_DEVICE(opaque) or PCI_DEVICE(d) after this
>>> patch. We know it's a PCIIDEState and has PCIDevice parent_obj field
>>> because we set the opaque pointer when adding this callback so I think
>>> this works and is the less expensive way. But feel free to change it any
>>> way you like and use it that way. I'd keep it as it is.
>>
>> My understanding of QOM is we shouldn't access internal states that
>> way, because 1/ this makes object refactors harder and 2/ this is
>> not the style/example we want in the codebase, but it doesn't seem
>> documented, so Cc'ing Markus/Eduardo for confirmation.
>
> `PCI_DEVICE(d)` is preferred instead `of d.parent_obj` (parent_obj is
> just a QOM implementation detail).  If there are real performance
> reasons to avoid it, we need numbers to support that.

OK I can try to do some measurement or go back to PCI_DEVICE() then.

> Also, note that `OBJECT_CHECK(obj)` is just `return obj` if
> CONFIG_QOM_CAST_DEBUG is disabled.

But configure enables it by default even without --enable-debug so I think 
most people don't even know and it's enabled practically everywhere 
(probably even in distros). Why can't we have it disabled by default if 
it's a developer debug option and enable it only for developers where 
it's useful to catch bugs?

Regards,
BALATON Zoltan