Re: [PATCH 3/3] drivers/vfio/pci: Fix MSIx message lost

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2014-03-03 at 15:43 +1100, Alexey Kardashevskiy wrote:
> While it works for our particular problem and seems correct, it has one
> flaw - hw/pci/msix.c will not generate this backtrace if masking bit does
> not change which can happen in general:
> ===
> static void msix_handle_mask_update(PCIDevice *dev, int vector, bool
> was_masked)
> {
>     bool is_masked = msix_is_masked(dev, vector);
> 
>     if (is_masked == was_masked) {
>         return;
>     }
> ===
> 
> Or if masking bit is the same, nothing bad is expected?...

Hrm ok, so it will work in this specific case but might not in the general
case of a driver triggering some kind of local reset on the device requiring
the MSI-X to be restored. The guest will write but qemu will swallow them ...

I think that needs to be fixed but it might be hard without introducing
a new ioctl from what I can see of the way the code is structured... Unless
qemu turns that into a disable/enable pair I suppose.

Cheers,
Ben.