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Wed, 20 Oct 2021 11:18:45 -0400 X-MC-Unique: YQj7d7A8McKRN3wRFitHnA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5346887950C; Wed, 20 Oct 2021 15:18:44 +0000 (UTC) Received: from localhost (unknown [10.22.17.166]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6D0B55F4E1; Wed, 20 Oct 2021 15:18:25 +0000 (UTC) Date: Wed, 20 Oct 2021 11:18:24 -0400 From: Eduardo Habkost To: BALATON Zoltan Subject: Re: [PATCH] via-ide: Avoid expensive operations in irq handler Message-ID: <20211020151824.ecl3etam425diyli@habkost.net> References: <20211018014059.13E65746353@zero.eik.bme.hu> <549ece11-990f-a19b-5dfe-508e315a6163@amsat.org> <31027ddc-b618-9628-d725-1516f7bfd098@amsat.org> <20211020143626.dvthmwizsljuwqz4@habkost.net> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=ehabkost@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=ehabkost@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Markus Armbruster , Huacai Chen , qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Gerd Hoffmann , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Oct 20, 2021 at 04:58:58PM +0200, BALATON Zoltan wrote: > 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 > > > > > > Signed-off-by: BALATON Zoltan > > > > > > --- > > > > > >  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? It's a trade-off, I don't think there's a right answer for everybody. Personally, I'd say the benefits outweigh the risks of disabling it for most users (especially the ones building QEMU directly from source). I don't mean to say it's OK for CONFIG_QOM_CAST_DEBUG=y builds to have visibly poor performance. If the typecast above causes a measurable performance impact, it might be reasonable to have a workaround like: static void via_ide_set_irq(void *opaque, int n, int level) { PCIIDEState *ide = opaque; PCIDevice *pci = opaque; ... } -- Eduardo