[RFC QEMU PATCH v4 0/1] Support device passthrough when dom0 is PVH on Xen

[Jiqian Chen <Jiqian.Chen@xxxxxxx>]

Hi All,
This is v4 series to support passthrough on Xen when dom0 is PVH.
v3->v4 changes:
* Add gsi into struct XenHostPCIDevice and use gsi number that read from gsi 
sysfs if it exists, if there is no gsi sysfs, still use irq.

v2->v3 changes:
* du to changes in the implementation of the second patch on kernel side(that 
adds a new sysfs for gsi instead of a new syscall), so read gsi number from the 
sysfs of gsi.

Below is the description of v2 cover letter:
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.

Jiqian Chen (1):
  xen: Use gsi instead of irq for mapping pirq

 hw/xen/xen-host-pci-device.c | 7 +++++++
 hw/xen/xen-host-pci-device.h | 1 +
 hw/xen/xen_pt.c              | 6 +++++-
 3 files changed, 13 insertions(+), 1 deletion(-)

-- 
2.34.1