On Fri, Dec 31, 2010 at 11:05:28AM +0800, Sheng Yang wrote: > 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 âpure pollingâ 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. Well, it doesn't seem to fit this part of the definition > > 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 âpure pollingâ mode of operation. looking at IRQ_PENDING will make the pending bit *never* clear while the vector is masked. > 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
