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

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

On Tue, May 05, 2009 at 07:49:10AM -0300, Marcelo Tosatti wrote:
> On Tue, May 05, 2009 at 01:34:50PM +0300, Michael S. Tsirkin wrote:
> > 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 ...
> 
> Because there is no registered (kvm) memory slot for the range which
> e1000 registers its MMIO? Not sure about the address of the MSI-X table
> page, but you could achieve the same effect by splitting the slot which
> it lives in two, with a 1 page hole between them.

You could also move the emulated MSI-X table, sticking it on top of the
existing BAR. Since PCI config includes the pointer to the table,
a driver that reads this pointer will continue to work.

Of course, there's no guarantee that guest drivers don't just hard-code
this offset.

> BTW this is why you can't map the MSI-X table page directly, you want
> accesses to be trapped.

BTW current design won't work if the base page size is > 4K, will it?
The hole covers a page, so you'll get faults outside the MSI-X table.

-- 
MST