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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.