On 06/07/2015 12:37, Christoffer Dall wrote:

I don't view it as 'the kernel requires this' but as 'the kernel will
not complain with arbitrary error code if you set the devid flag'
capability, and it's up to userspace (as usual) to provide the correct
arguments for things to work, and up to the kernel to ensure we don't
crash the system etc.

Thus, if you want to advertise it as a capability, I would rather call
it KVM_CAP_MSI_DEVID.

I agree.  Does userspace know that ITS guests always require devid?  I
guess it's okay to return -EINVAL if the userspace doesn't set the flag
but the virtual hardware requires it.

Paolo

The question is if userspace code that sets the devid flag will anyway
depend on some discovery mechanism of whether or not the kernel supports
arm64 irqfd etc. and if so, can we be sure to add the required support
at once in the kernel so that EINVAL never means 'you set the flags
field on the ioctl on an old kernel'?

This smells an awful lot like a capability to me.