Re: qemu/hw/device-assignment: questions about msix_table_page

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

On Tue, May 05, 2009 at 07:19:45AM -0300, Marcelo Tosatti wrote:
> On Tue, May 05, 2009 at 12:51:36PM +0300, Michael S. Tsirkin wrote:
> > On Mon, Apr 27, 2009 at 10:30:17PM +0800, Sheng Yang wrote:
> > > > > > > If guest can write to the real device MSI-X table directly, it would
> > > > > > > cause chaos on interrupt delivery, for what guest see is totally
> > > > > > > different with what's host see...
> > > > > >
> > > > > > Obviously.
> > > > > >
> > > > > > Thanks,
> > > 
> > 
> > What's the reason that this page is unmapped from the qemu memory space?
> > Specifically what do these lines do:
> >             int offset = r_dev->msix_table_addr - real_region->base_addr;
> >             ret = munmap(region->u.r_virtbase + offset, TARGET_PAGE_SIZE);
> 
> I believe this allows accesses to this page (the MSI-X table), which
> is part of the guest address space (through kvm memory slots), to be
> trapped by qemu.
> 
> Since there is no actual page in this guest address, KVM treats accesses
> as MMIO and forwards them to QEMU.
> 
> 

I thought about this too.
But why is this necessary for assigned MSI-X but not for emulated devices such as
e.g. e1000? All e1000 does seems to be cpu_register_physical_memory ...

-- 
MST