Re: [XEN PATCH v13 3/6] x86/pvh: Add PHYSDEVOP_setup_gsi for PVH dom0

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 20 Aug 2024, Chen, Jiqian wrote:
> On 2024/8/19 17:16, Jan Beulich wrote:
> > On 16.08.2024 13:08, Jiqian Chen wrote:
> >> The gsi of a passthrough device must be configured for it to be
> >> able to be mapped into a hvm domU.
> >> But When dom0 is PVH, the gsis may not get registered(see below
> >> clarification), it causes the info of apic, pin and irq not be
> >> added into irq_2_pin list, and the handler of irq_desc is not set,
> >> then when passthrough a device, setting ioapic affinity and vector
> >> will fail.
> >>
> >> To fix above problem, on Linux kernel side, a new code will
> >> need to call PHYSDEVOP_setup_gsi for passthrough devices to
> >> register gsi when dom0 is PVH.
> >>
> >> So, add PHYSDEVOP_setup_gsi into hvm_physdev_op for above
> >> purpose.
> >>
> >> Clarify two questions:
> >> First, why the gsi of devices belong to PVH dom0 can work?
> >> Because when probe a driver to a normal device, it uses the normal
> >> probe function of pci device, in its callstack, it requests irq
> >> and unmask corresponding ioapic of gsi, then trap into xen and
> >> register gsi finally.
> >> Callstack is(on linux kernel side) pci_device_probe->
> >> request_threaded_irq-> irq_startup-> __unmask_ioapic->
> >> io_apic_write, then trap into xen hvmemul_do_io->
> >> hvm_io_intercept-> hvm_process_io_intercept->
> >> vioapic_write_indirect-> vioapic_hwdom_map_gsi-> mp_register_gsi.
> >> So that the gsi can be registered.
> >>
> >> Second, why the gsi of passthrough device can't work when dom0
> >> is PVH?
> >> Because when assign a device to passthrough, it uses the specific
> >> probe function of pciback, in its callstack, it doesn't install a
> >> fake irq handler due to the ISR is not running. So that
> >> mp_register_gsi on Xen side is never called, then the gsi is not
> >> registered.
> >> Callstack is(on linux kernel side) pcistub_probe->pcistub_seize->
> >> pcistub_init_device-> xen_pcibk_reset_device->
> >> xen_pcibk_control_isr->isr_on==0.
> > 
> > So: Underlying XSA-461 was the observation that the very limited set of
> > cases where this fake IRQ handler is installed is an issue. The problem
> > of dealing with "false" IRQs when a line-based interrupt is shared
> > between devices affects all parties, not just Dom0 and not just PV
> > guests. Therefore an alternative to the introduction of a new hypercall
> > would be to simply leverage that the installation of such a handler
> > will need widening anyway.
> > 
> > However, the installation of said handler presently also occurs in
> > cases where it's not really needed - when the line isn't shared. Thus,
> > if the handler registration would also be eliminated when it's not
> > really needed, we'd be back to needing a separate hypercall.
> > 
> > So I think first of all it needs deciding what is going to be done in
> > Linux, at least in pciback (as here we care about the Dom0 case only).
> Agree, so the current options are either to use hypercall 
> (PHYSDEVOP_setup_gsi) or to install fake IRQ handler in pciback.
> So, we may need the inputs from the Maintainers on Linux side.
> Hi Stefano and Juergen, what about your opinions?

I would go with the PHYSDEVOP_setup_gsi solution