Re: [PATCH 0/8][v2] MSI-X mask emulation support for assigned device

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 21, 2010 at 03:10:19PM +0800, Sheng Yang wrote:
> On Thursday 21 October 2010 03:02:24 Marcelo Tosatti wrote:
> > On Wed, Oct 20, 2010 at 04:26:24PM +0800, Sheng Yang wrote:
> > > Here is v2.
> > > 
> > > Changelog:
> > > 
> > > v1->v2
> > > 
> > > The major change from v1 is I've added the in-kernel MSI-X mask emulation
> > > support, as well as adding shortcuts for reading MSI-X table.
> > > 
> > > I've taken Michael's advice to use mask/unmask directly, but unsure about
> > > exporting irq_to_desc() for module...
> > > 
> > > Also add flush_work() according to Marcelo's comments.
> > > 
> > > Sheng Yang (8):
> > >   PCI: MSI: Move MSI-X entry definition to pci_regs.h
> > >   irq: Export irq_to_desc() to modules
> > >   KVM: x86: Enable ENABLE_CAP capability for x86
> > >   KVM: Move struct kvm_io_device to kvm_host.h
> > >   KVM: Add kvm_get_irq_routing_entry() func
> > >   KVM: assigned dev: Preparation for mask support in userspace
> > >   KVM: assigned dev: Introduce io_device for MSI-X MMIO accessing
> > >   KVM: Emulation MSI-X mask bits for assigned devices
> > 
> > Why does the current scheme, without msix per-vector mask support, is
> > functional at all? Luck?
> 
> Well, I believe we are lucky... We just ignored the operation in the past.
> 
> I had raised this issue when Michael begin to work on MSI-X support in QEmu long 
> ago, but then I was busy on some other things. Until now when Eddie want to add 
> MSI-X in-kernel acceleration, we back to it...
> 
> And about the "flush_work()" you commented, I still think even for the native 
> device, it's possible that the short time after OS write to the mask bit, the 
> interrupt may be delivered, if the device already send the message out on the 
> bus(I just guess, haven't observed)... The spec didn't say that the finish of 
> writing mask bit behavior would also get all message on the bus delivered. So I 
> think leave the work a little late would also be fine.

Yes ... but e.g. bus read afterwards would have to flush the
write up the bus...

> --
> regards
> Yang, Sheng