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

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

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?

Thanks, Roger.