On 06/07/15 13:08, Christoffer Dall wrote:

On Mon, Jul 06, 2015 at 12:23:19PM +0100, Andre Przywara wrote:

quoted

Hi Paolo,

thanks for looking at this!

On 06/07/15 12:07, Paolo Bonzini wrote:

quoted

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

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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?

Well, as we are about to implement this: yes. But the issue is that MSI
injection and GSI routing code is generic PCI code in userland (at least
in kvmtool, guess in QEMU, too), so I don't want to pull in any kind of
ARM specific code in there. The idea is to always provide the device ID
from the PCI code (for PCI devices it's just the B/D/F triplet), but
only send it to the kernel if needed. Querying a KVM capability is
perfectly fine for this IMO.

quoted

I
guess it's okay to return -EINVAL if the userspace doesn't set the flag
but the virtual hardware requires it.

Yes, that is what I do in the kernel implementation. And that is
perfectly fine: the ITS emulation does not work without a device ID, the
ITS driver in the guest assigns the very same payload (and address) to
different devices, so there is no way to tell the MSIs apart without a
unique device ID.

Just so I'm sure I understand: The way the kernel differentiates between
no-devid and devid==0, is whether or not the devid flag is set, correct?

Yes, that is the idea. The plan for the implementation is like this:
1) If the kernel does not need the device ID (x86, GICv2M), it does not
care about the flag or the value at all.
2) In case for ITS on ARM64, the kernel returns an error is the flag is
not set.

Cheers,
Andre.