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

Avi Kivity
2011-01-02T10:58:55Z

On 01/02/2011 12:39 PM, Michael S. Tsirkin wrote:
> > >
> > >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?
>
> I don't see how userspace can send interrupts with this
> interface unfortunately. We also need irqfd support ...

Sure we'll need additions to that interface.

What about vhost-net and vfio? I thought that they could emulate the
mask bits:

- KVM_MMIOFD(vmfd, mmio_range, fd1, fd2) associates an mmio range with an fd
- writel(mmio_range) or readl(mmio_range) from the guest causes a
command to be written to fd1
- for readl(), read from fd2 to see the result (works nicely for "pci
read flushes posted writes")

this allows interesting stuff to be implemented in separate processes,
threads, or kernel modules.

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