[RFC QEMU PATCH v2 0/1] Support device passthrough when dom0 is PVH on Xen

[Jiqian Chen <Jiqian.Chen@xxxxxxx>]

Hi All,

This patch is the v2 of the implementation of passthrough when dom0 is PVH on 
Xen.

Issues we encountered:
1. failed to map pirq for gsi
Problem: qemu will call xc_physdev_map_pirq() to map a passthrough device’s gsi 
to pirq in function
xen_pt_realize(). But failed.

Reason: According to the implement of xc_physdev_map_pirq(), it needs gsi 
instead of irq, but qemu
pass irq to it and treat irq as gsi, it is got from file 
/sys/bus/pci/devices/xxxx:xx:xx.x/irq in
function xen_host_pci_device_get(). But actually the gsi number is not equal 
with irq. On PVH dom0,
when it allocates irq for a gsi in function acpi_register_gsi_ioapic(), 
allocation is dynamic, and
follow the principle of applying first, distributing first. And if you debug 
the kernel codes(see
function __irq_alloc_descs), you will find the irq number is allocated from 
small to large by order,
but the applying gsi number is not, gsi 38 may come before gsi 28, that causes 
gsi 38 get a smaller
irq number than gsi 28, and then gsi != irq.

Solution: we can record the relation between gsi and irq, then when 
userspace(qemu) want to use gsi,
we can do a translation. The third patch of kernel(xen/privcmd: Add new syscall 
to get gsi from irq)
records all the relations in acpi_register_gsi_xen_pvh() when dom0 initialize 
pci devices, and provide
a syscall for userspace to get the gsi from irq. The third patch of xen(tools: 
Add new function to get
gsi from irq) add a new function xc_physdev_gsi_from_irq() to call the new 
syscall added on kernel side.
And then userspace can use that function to get gsi. Then xc_physdev_map_pirq() 
will success.

This v2 on qemu side is the same as the v1 (
qemu 
https://lore.kernel.org/xen-devel/20230312092244.451465-19-ray.huang@xxxxxxx/), 
just call
xc_physdev_gsi_from_irq() to get gsi from irq.

v2 on kernel side:
https://lore.kernel.org/lkml/20231124103123.3263471-1-Jiqian.Chen@xxxxxxx/T/#t

v2 on Xen side:
https://lore.kernel.org/xen-devel/20231124104136.3263722-1-Jiqian.Chen@xxxxxxx/T/#t

Jiqian Chen (1):
  xen/pci: get gsi from irq for passthrough devices

 hw/xen/xen-host-pci-device.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

-- 
2.34.1