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

Sheng Yang
2010-12-31T03:05:46Z

On Thursday 30 December 2010 18:32:56 Michael S. Tsirkin wrote:
> 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).

For assigned device, I think the result we should return is IRQ_PENDING bit of
related IRQ. Seems it perfectly fits the meaning of pending bit definition here -
set when masked, and if we didn't clean it, one interrupt would be retriggered
after unmask. But it's a internal flag, and use it would lead to some core
change(more need to be considered if we want to operate the flag bit outside core
kernel part).
>
> 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.

I agree. At least it's not a regression. And in fact we haven't seen any device
driver use this. I've checked Linux kernel code, found no one used PCI_MSIX_PBA or
msix_pba_offset_reg().

I guess it's fine to get MSI-X mask part in first, then deal with PBA part if
necessary - though we haven't seen any driver use it so far. It won't be worse
with this patch anyway...

--
regards
Yang, Sheng