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?
> > Jan -- Best regards, Jiqian Chen.
