RE: [RFC PATCH] vfio/pci: add PCIe TPH to device feature ioctl

[Wathsala Wathawana Vithanage]

> Having said that, regardless of this proposal or the availability of kernel
> TPH support, a VFIO driver could enable TPH and set an arbitrary ST on the
> MSI-X/ST table or a device-specific location on supported platforms. If the
> driver doesn't have a list of valid STs, it can enumerate 8- or 16-bit STs and
> measure access latencies to determine valid ones.
> 
I tested enabling TPH inside a VM with setpci, it turned out that it doesn't
write to TPH control register thankfully because vfio_pci_init_perm_bits()
in host kernel is not setting config write permissions for PCI_EXT_CAP_ID_TPH. 

QEMU traps config writes and routes them via vfio_pci_write_config().
So, like how it already handles MSI/MSI-X enablement, TPH enablement
too could be handled by a special handler that does the following.
1. Error out on device specific mode enablement.
2. Check ST-table writes for STs that do not belong to CPUs the VM is bound to.

Then for MSI-X mode, a Quirk could trap and check STs for the #2 case
above.
To make #1 above consistent with TPH cap register, some emulated bits can
be added to mask out availability of device specific mode.

--wathsala