Re: [RFC XEN PATCH v2 3/3] tools: Add new function to get gsi from irq

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 30, 2023 at 07:09:12PM -0800, Stefano Stabellini wrote:
> On Thu, 30 Nov 2023, Roger Pau Monné wrote:
> > On Wed, Nov 29, 2023 at 08:02:40PM -0800, Stefano Stabellini wrote:
> > > n Wed, 29 Nov 2023, Stefano Stabellini wrote:
> > > > On Tue, 28 Nov 2023, Roger Pau Monné wrote:
> > > > > On Fri, Nov 24, 2023 at 06:41:36PM +0800, Jiqian Chen wrote:
> > > > > > In PVH dom0, it uses the linux local interrupt mechanism,
> > > > > > when it allocs irq for a gsi, it is dynamic, and follow
> > > > > > the principle of applying first, distributing first. And
> > > > > > if you debug the kernel codes, you will find the irq
> > > > > > number is alloced from small to large, but the applying
> > > > > > gsi number is not, may gsi 38 comes before gsi 28, that
> > > > > > causes the irq number is not equal with the gsi number.
> > > > > > And when we passthrough a device, QEMU will use its gsi
> > > > > > number to do mapping actions, see xen_pt_realize->
> > > > > > xc_physdev_map_pirq, but the gsi number is got from file
> > > > > > /sys/bus/pci/devices/xxxx:xx:xx.x/irq in current code,
> > > > > > so it will fail when mapping.
> > > > > > And in current codes, there is no method to translate
> > > > > > irq to gsi for userspace.
> > > > > 
> > > > > I think it would be cleaner to just introduce a new sysfs node that
> > > > > contains the gsi if a device is using one (much like the irq sysfs
> > > > > node)?
> > > > > 
> > > > > Such ioctl to translate from IRQ to GSI has nothing to do with Xen, so
> > > > > placing it in privcmd does seem quite strange to me.  I understand
> > > > > that for passthrough we need the GSI, but such translation layer from
> > > > > IRQ to GSI is all Linux internal, and it would be much simpler to just
> > > > > expose the GSI in sysfs IMO.
> > > > 
> > > > Maybe something to add to drivers/xen/sys-hypervisor.c in Linux.
> > > > Juergen what do you think?
> > > 
> > > Let me also add that privcmd.c is already a Linux specific interface.
> > > Although it was born to be a Xen hypercall "proxy" in reality today we
> > > have a few privcmd ioctls that don't translate into hypercalls. So I
> > > don't think that adding IOCTL_PRIVCMD_GSI_FROM_IRQ would be a problem.
> > 
> > Maybe not all ioctls translate to hypercalls (I guess you are
> > referring to the IOCTL_PRIVCMD_RESTRICT ioctl), but they are specific
> > Xen actions.  Getting the GSI used by a device has nothing do to with
> > Xen.
> > 
> > IMO drivers/xen/sys-hypervisor.c is also not appropriate, but I'm not
> > the maintainer of any of those components.
> > 
> > There's nothing Xen specific about fetching the GSI associated with a
> > PCI device.  The fact that Xen needs it for passthrough is just a red
> > herring, further cases where the GSI is needed might arise outside of
> > Xen, and hence such node would better be placed in a generic
> > location.  The right location should be /sys/bus/pci/devices/<sbdf>/gsi.
> 
> That might be true but /sys/bus/pci/devices/<sbdf>/gsi is a non-Xen
> generic interface and the maintainers of that portion of Linux code
> might have a different opinion. We'll have to see.

Right, but before resorting to implement a Xen specific workaround
let's attempt to do it the proper way :).

I cannot see why exposing the gsi on sysfs like that would be an
issue.  There's a lot of resource information exposed on sysfs
already, and it's a trivial node to implement.

Thanks, Roger.