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

Avi Kivity
2010-12-30T11:27:20Z

On 12/30/2010 01:07 PM, Michael S. Tsirkin wrote:
> 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'

But when we set the mask bit, it must change the pending bit back to the
function-specific condition?

> > >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.

If interrupt condition exists, and the vector is masked, the pending bit
is set. Otherwise the pending bit is clear. Better?

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