Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

From: Bjorn Helgaas <helgaas@kernel.org>
To: Zhangfei Gao <zhangfei.gao@linaro.org>
Cc: linux-pci <linux-pci@vger.kernel.org>,
	Hanjun Guo <guohanjun@huawei.com>,
	jean-philippe <jean-philippe@linaro.org>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Len Brown <lenb@kernel.org>,
	Thanu Rangarajan <Thanu.Rangarajan@arm.com>,
	Souvik Chakravarty <Souvik.Chakravarty@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Bjorn Helgaas <bhelgaas@google.com>,
	wanghuiqiang <wanghuiqiang@huawei.com>,
	kenneth-lee-2012@foxmail.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"open list:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
	"open list:HARDWARE RANDOM NUMBER GENERATOR CORE"
	<linux-crypto@vger.kernel.org>,
	Sudeep Holla <sudeep.holla@arm.com>
Subject: Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU
Date: Tue, 23 Jun 2020 10:04:27 -0500	[thread overview]
Message-ID: <20200623150427.GA2403606@bjorn-Precision-5520> (raw)
In-Reply-To: <c5d7b2f1-6b32-d965-3b60-eb70a26e02b4@linaro.org>

On Fri, Jun 19, 2020 at 10:26:54AM +0800, Zhangfei Gao wrote:
> Have studied _DSM method, two issues we met comparing using quirk.
> 
> 1. Need change definition of either pci_host_bridge or pci_dev, like adding
> member can_stall,
> while pci system does not know stall now.
> 
> a, pci devices do not have uuid: uuid need be described in dsdt, while pci
> devices are not defined in dsdt.
>     so we have to use host bridge.

PCI devices *can* be described in the DSDT.  IIUC these particular
devices are hardwired (not plug-in cards), so platform firmware can
know about them and could describe them in the DSDT.

> b,  Parsing dsdt is in in pci subsystem.
> Like drivers/acpi/pci_root.c:
>        obj = acpi_evaluate_dsm(ACPI_HANDLE(bus->bridge), &pci_acpi_dsm_guid,
> 1,
>                                 IGNORE_PCI_BOOT_CONFIG_DSM, NULL);
> 
> After parsing DSM in pci, we need record this info.
> Currently, can_stall info is recorded in iommu_fwspec,
> which is allocated in iommu_fwspec_init and called by iort_iommu_configure
> for uefi.

You can look for a _DSM wherever it is convenient for you.  It could
be in an AMBA shim layer.

> 2. Guest kernel also need support sva.
> Using quirk, the guest can boot with sva enabled, since quirk is
> self-contained by kernel.
> If using  _DSM, a specific uefi or dtb has to be provided,
> currently we can useQEMU_EFI.fd from apt install qemu-efi

I don't quite understand what this means, but as I mentioned before, a
quirk for a *limited* number of devices is OK, as long as there is a
plan that removes the need for a quirk for future devices.

E.g., if the next platform version ships with a DTB or firmware with a
_DSM or other mechanism that enables the kernel to discover this
information without a kernel change, it's fine to use a quirk to cover
the early platform.

The principles are:

  - I don't want to have to update a quirk for every new Device ID
    that needs this.

  - I don't really want to have to manage non-PCI information in the
    struct pci_dev.  If this is AMBA- or IOMMU-related, it should be
    stored in a structure related to AMBA or the IOMMU.
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