Re: [Qemu-devel] [RFC PATCH V2 02/10] Qemu/VFIO: Add new VFIO_GET_PCI_CAP_INFO ioctl cmd definition

[Lan, Tianyu]

On 12/3/2015 6:25 AM, Alex Williamson wrote:
> I didn't seen a matching kernel patch series for this, but why is the
> kernel more capable of doing this than userspace is already?
The following link is the kernel patch.
http://marc.info/?l=kvm&m=144837328920989&w=2

> These seem
> like pointless ioctls, we're creating a purely virtual PCI capability,
> the kernel doesn't really need to participate in that.

VFIO kernel driver has pci_config_map which indicates the PCI capability 
position and length which helps to find free PCI config regs. Qemu side 
doesn't have such info and can't get the exact table size of PCI 
capability. If we want to add such support in the Qemu, needs duplicates 
a lot of code of vfio_pci_configs.c in the Qemu.

> Also, why are we
> restricting ourselves to standard capabilities?

This version is to check whether it's on the right way and We can extend
this to pci extended capability later.

> That's often a crowded
> space and we can't always know whether an area is free or not based only
> on it being covered by a capability.  Some capabilities can also appear
> more than once, so there's context that isn't being passed to the kernel
> here.  Thanks,

The region outside of PCI capability are not passed to kernel or used by 
Qemu for MSI/MSIX . It's possible to use these places for new 
capability. One concerns is that guest driver may abuse them and quirk 
of masking some special regs outside of capability maybe helpful.