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

[Eduardo Habkost]

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.

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

-- 
Eduardo