Received-SPF: pass (google.com: domain of linux-crypto-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 1 Jun 2020 12:41:04 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Zhangfei Gao <zhangfei.gao@linaro.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Hanjun Guo <guohanjun@huawei.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>,
        jean-philippe <jean-philippe@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        kenneth-lee-2012@foxmail.com, Wangzhou <wangzhou1@hisilicon.com>,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        iommu@lists.linux-foundation.org, linux-acpi@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org
Subject: Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU
Message-ID: <20200601174104.GA734973@bjorn-Precision-5520>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200528073344.GO5221@8bytes.org>
Sender: linux-crypto-owner@vger.kernel.org
Precedence: bulk

On Thu, May 28, 2020 at 09:33:44AM +0200, Joerg Roedel wrote:
> On Wed, May 27, 2020 at 01:18:42PM -0500, Bjorn Helgaas wrote:
> > Is this slowdown significant?  We already iterate over every device
> > when applying PCI_FIXUP_FINAL quirks, so if we used the existing
> > PCI_FIXUP_FINAL, we wouldn't be adding a new loop.  We would only be
> > adding two more iterations to the loop in pci_do_fixups() that tries
> > to match quirks against the current device.  I doubt that would be a
> > measurable slowdown.
> 
> I don't know how significant it is, but I remember people complaining
> about adding new PCI quirks because it takes too long for them to run
> them all. That was in the discussion about the quirk disabling ATS on
> AMD Stoney systems.
> 
> So it probably depends on how many PCI devices are in the system whether
> it causes any measureable slowdown.

I found this [1] from Paul Menzel, which was a slowdown caused by
quirk_usb_early_handoff().  I think the real problem is individual
quirks that take a long time.

The PCI_FIXUP_IOMMU things we're talking about should be fast, and of
course, they're only run for matching devices anyway.  So I'd rather
keep them as PCI_FIXUP_FINAL than add a whole new phase.

Bjorn

[1] https://lore.kernel.org/linux-pci/b1533fd5-1fae-7256-9597-36d3d5de9d2a@molgen.mpg.de/