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

[Sheng Yang <sheng@linux.intel.com>]

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