Re: [PATCH 2/2][RFC] KVM: Emulate MSI-X table and PBA in kernel

Michael S. Tsirkin
2010-12-30T10:33:17Z

On Thu, Dec 30, 2010 at 11:30:12AM +0200, Avi Kivity wrote:
> On 12/30/2010 09:47 AM, Michael S. Tsirkin wrote:
> >I am not really suggesting this. What I say is PBA is unimplemented
> >let us not commit to an interface yet.
>
> What happens to a guest that tries to use PBA?
> It's a mandatory part of MSI-X, no?

Yes. Unfortunately the pending bit is in fact a communication channel
used for function specific purposes when mask bit is set,
and 0 when unset. The spec even seems to *require* this use:

I refer to this:

For MSI and MSI-X, while a vector is masked, the function is prohibited
from sending the associated message, and the function must set the
associated Pending bit whenever the function would otherwise send the
message. When software unmasks a vector whose associated Pending bit is
set, the function must schedule sending the associated message, and
clear the Pending bit as soon as the message has been sent. Note that
clearing the MSI-X Function Mask bit may result in many messages needing
to be sent.

If a masked vector has its Pending bit set, and the associated
underlying interrupt events are somehow satisfied (usually by software
though the exact manner is function-specific), the function must clear
the Pending bit, to avoid sending a spurious interrupt message later
when software unmasks the vector. However, if a subsequent interrupt
event occurs while the vector is still masked, the function must again
set the Pending bit.

Software is permitted to mask one or more vectors indefinitely, and
service their associated interrupt events strictly based on polling
their Pending bits. A function must set and clear its Pending bits as
necessary to support this “pure polling” mode of operation.

For assigned devices, supporting this would require
that the mask bits on the device are set if the mask bit in
guest is set (otherwise pending bits are disabled).

Existing code does not support PBA in assigned devices, so at least it's
not a regression there, and the virtio spec says nothing about this so
we should be fine.

> --
> error compiling committee.c: too many arguments to function