Re: [RFC KERNEL PATCH v4 3/3] PCI/sysfs: Add gsi sysfs for pci_dev

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Feb 09, 2024 at 03:05:49PM -0600, Bjorn Helgaas wrote:
> On Thu, Feb 01, 2024 at 09:39:49AM +0100, Roger Pau Monné wrote:
> > On Wed, Jan 31, 2024 at 01:00:14PM -0600, Bjorn Helgaas wrote:
> > > On Wed, Jan 31, 2024 at 09:58:19AM +0100, Roger Pau Monné wrote:
> > > > On Tue, Jan 30, 2024 at 02:44:03PM -0600, Bjorn Helgaas wrote:
> > > > > On Tue, Jan 30, 2024 at 10:07:36AM +0100, Roger Pau Monné wrote:
> > > > > > On Mon, Jan 29, 2024 at 04:01:13PM -0600, Bjorn Helgaas wrote:
> > > > > > > On Thu, Jan 25, 2024 at 07:17:24AM +0000, Chen, Jiqian wrote:
> > > > > > > > On 2024/1/24 00:02, Bjorn Helgaas wrote:
> > > > > > > > > On Tue, Jan 23, 2024 at 10:13:52AM +0000, Chen, Jiqian wrote:
> > > > > > > > >> On 2024/1/23 07:37, Bjorn Helgaas wrote:
> > > > > > > > >>> On Fri, Jan 05, 2024 at 02:22:17PM +0800, Jiqian Chen wrote:
> > > > > > > > >>>> There is a need for some scenarios to use gsi sysfs.
> > > > > > > > >>>> For example, when xen passthrough a device to dumU, it will
> > > > > > > > >>>> use gsi to map pirq, but currently userspace can't get gsi
> > > > > > > > >>>> number.
> > > > > > > > >>>> So, add gsi sysfs for that and for other potential scenarios.
> > > > > > > > >> ...
> > > > > > > > > 
> > > > > > > > >>> I don't know enough about Xen to know why it needs the GSI in
> > > > > > > > >>> userspace.  Is this passthrough brand new functionality that can't be
> > > > > > > > >>> done today because we don't expose the GSI yet?
> > > > > > > 
> > > > > > > I assume this must be new functionality, i.e., this kind of
> > > > > > > passthrough does not work today, right?
> > > > > > > 
> > > > > > > > >> has ACPI support and is responsible for detecting and controlling
> > > > > > > > >> the hardware, also it performs privileged operations such as the
> > > > > > > > >> creation of normal (unprivileged) domains DomUs. When we give to a
> > > > > > > > >> DomU direct access to a device, we need also to route the physical
> > > > > > > > >> interrupts to the DomU. In order to do so Xen needs to setup and map
> > > > > > > > >> the interrupts appropriately.
> > > > > > > > > 
> > > > > > > > > What kernel interfaces are used for this setup and mapping?
> > > > > > > >
> > > > > > > > For passthrough devices, the setup and mapping of routing physical
> > > > > > > > interrupts to DomU are done on Xen hypervisor side, hypervisor only
> > > > > > > > need userspace to provide the GSI info, see Xen code:
> > > > > > > > xc_physdev_map_pirq require GSI and then will call hypercall to pass
> > > > > > > > GSI into hypervisor and then hypervisor will do the mapping and
> > > > > > > > routing, kernel doesn't do the setup and mapping.
> > > > > > > 
> > > > > > > So we have to expose the GSI to userspace not because userspace itself
> > > > > > > uses it, but so userspace can turn around and pass it back into the
> > > > > > > kernel?
> > > > > > 
> > > > > > No, the point is to pass it back to Xen, which doesn't know the
> > > > > > mapping between GSIs and PCI devices because it can't execute the ACPI
> > > > > > AML resource methods that provide such information.
> > > > > > 
> > > > > > The (Linux) kernel is just a proxy that forwards the hypercalls from
> > > > > > user-space tools into Xen.
> > > > > 
> > > > > But I guess Xen knows how to interpret a GSI even though it doesn't
> > > > > have access to AML?
> > > > 
> > > > On x86 Xen does know how to map a GSI into an IO-APIC pin, in order
> > > > configure the RTE as requested.
> > > 
> > > IIUC, mapping a GSI to an IO-APIC pin requires information from the
> > > MADT.  So I guess Xen does use the static ACPI tables, but not the AML
> > > _PRT methods that would connect a GSI with a PCI device?
> > 
> > Yes, Xen can parse the static tables, and knows the base GSI of
> > IO-APICs from the MADT.
> > 
> > > I guess this means Xen would not be able to deal with _MAT methods,
> > > which also contains MADT entries?  I don't know the implications of
> > > this -- maybe it means Xen might not be able to use with hot-added
> > > devices?
> > 
> > It's my understanding _MAT will only be present on some very specific
> > devices (IO-APIC or CPU objects).  Xen doesn't support hotplug of
> > IO-APICs, but hotplug of CPUs should in principle be supported with
> > cooperation from the control domain OS (albeit it's not something that
> > we tests on our CI).  I don't expect however that a CPU object _MAT
> > method will return IO APIC entries.
> > 
> > > The tables (including DSDT and SSDTS that contain the AML) are exposed
> > > to userspace via /sys/firmware/acpi/tables/, but of course that
> > > doesn't mean Xen knows how to interpret the AML, and even if it did,
> > > Xen probably wouldn't be able to *evaluate* it since that could
> > > conflict with the host kernel's use of AML.
> > 
> > Indeed, there can only be a single OSPM, and that's the dom0 OS (Linux
> > in our context).
> > 
> > Getting back to our context though, what would be a suitable place for
> > exposing the GSI assigned to each device?
> 
> IIUC, the Xen hypervisor:
> 
>   - Interprets /sys/firmware/acpi/tables/APIC (or gets this via
>     something running on the Dom0 kernel) to find the physical base
>     address and GSI base, e.g., from I/O APIC, I/O SAPIC.

No, Xen parses the MADT directly from memory, before stating dom0.
That's a static table so it's fine for Xen to parse it and obtain the
I/O APIC GSI base.

>   - Needs the GSI to locate the APIC and pin within the APIC.  The
>     Dom0 kernel is the OSPM, so only it can evaluate the AML _PRT to
>     learn the PCI device -> GSI mapping.

Yes, Xen doesn't know the PCI device -> GSI mapping.  Dom0 needs to
parse the ACPI methods and signal Xen to configure a GSI with a
given trigger and polarity.

>   - Has direct access to the APIC physical base address to program the
>     Redirection Table.

Yes, the hardware (native) I/O APIC is owned by Xen, and not directly
accessible by dom0.

> The patch seems a little messy to me because the PCI core has to keep
> track of the GSI even though it doesn't need it itself.  And the
> current patch exposes it on all arches, even non-ACPI ones or when
> ACPI is disabled (easily fixable).
> 
> We only call acpi_pci_irq_enable() in the pci_enable_device() path, so
> we don't know the GSI unless a Dom0 driver has claimed the device and
> called pci_enable_device() for it, which seems like it might not be
> desirable.

I think that's always the case, as on dom0 devices to be passed
through are handled by pciback which does enable them.

I agree it might be best to not tie exposing the node to
pci_enable_device() having been called.  Is _PRT only evaluated as
part of acpi_pci_irq_enable()? (or pci_enable_device()).

> I was hoping we could put it in /sys/firmware/acpi/interrupts, but
> that looks like it's only for SCI statistics.  I guess we could moot a
> new /sys/firmware/acpi/gsi/ directory, but then each file there would
> have to identify a device, which might not be as convenient as the
> /sys/devices/ directory that already exists.  I guess there may be
> GSIs for things other than PCI devices; will you ever care about any
> of those?

We only support passthrough of PCI devices so far, but I guess if any
of such non-PCI devices ever appear and those use a GSI, and Xen
supports passthrough for them, then yes, we would need to fetch such
GSI somehow.

Thanks, Roger.