Re: [XEN PATCH v12 3/7] x86/pvh: Add PHYSDEVOP_setup_gsi for PVH dom0

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 8 Jul 2024, 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.
> 
> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>
> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>  xen/arch/x86/hvm/hypercall.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/xen/arch/x86/hvm/hypercall.c b/xen/arch/x86/hvm/hypercall.c
> index 03ada3c880bd..cfe82d0f96ed 100644
> --- a/xen/arch/x86/hvm/hypercall.c
> +++ b/xen/arch/x86/hvm/hypercall.c
> @@ -86,6 +86,7 @@ long hvm_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) 
> arg)
>              return -ENOSYS;
>          break;
>  
> +    case PHYSDEVOP_setup_gsi:
>      case PHYSDEVOP_pci_mmcfg_reserved:
>      case PHYSDEVOP_pci_device_add:
>      case PHYSDEVOP_pci_device_remove:
> -- 
> 2.34.1
>