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

Michael S. Tsirkin
2010-12-30T11:08:30Z

On Thu, Dec 30, 2010 at 12:37:18PM +0200, Avi Kivity wrote:
> On 12/30/2010 12:32 PM, 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).
>
> Can't this be done by setting the real mask bit when the guest reads
> the virtual pending bit, then reading the real pending bit?

Function specific is function-specific, but most likely not,
by that time the pending bit in the device might be clear:
'clear the Pending bit as soon as the message has been sent'

> >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.
>
> Why isn't it subject to the pci spec?
>
> If an interrupt condition exits, the bit should be set.

I wish. But this is not what the spec says above. It says if vector is
unmasked, bit must be cleared.

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