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

Avi Kivity
2011-01-02T10:26:16Z

On 12/31/2010 05:05 AM, Sheng Yang wrote:
> >
> > 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...

In a way it is worse because before, the fix would belong in user space,
which is easier to test and distribute. Now we have to fix it in the
kernel.

However I recognize that drivers which rely on the pending bit are
rare/nonexistent (likely on in preboot environments where interrupts are
hard), so even if we do code it, it will likely be incorrect (certainly
without a test).

So I'll accept the patch without PBA. Michael, what about supporting
virtio? Can we base something on this patch?

--
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