Re: [Xen-devel] [PATCH][v2] Hybrid extension support in Xen

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2010-02-02 at 13:28 +0000, Sheng Yang wrote:
> On Tuesday 02 February 2010 21:19:32 Ian Campbell wrote:
> > On Tue, 2010-02-02 at 12:54 +0000, Sheng Yang wrote:
> > > On Tuesday 02 February 2010 19:22:21 Ian Campbell wrote:
> > > > On Tue, 2010-02-02 at 08:16 +0000, Sheng Yang wrote:
> > > > > +/* Reserve 128KB for grant table */
> > > > > +#define GNTTAB_MEMBASE     0xfbfe0000
> > > > > +#define GNTTAB_MEMSIZE     0x20000
> > > >
> > > > Why is this necessary? Isn't the grant table contained within one of
> > > > the BARS on the virtual PCI device? What needs grant tables for prior
> > > > to the kernel finding the PCI device which necessitates hardcoding
> > > > these addresses in both guest and hypervisor?
> > >
> > > Thanks for so quick and detail comments. :)
> > >
> > > And this one is purposed, because we don't want to depends on QEmu. As
> > > you see, we now have PV drivers, QEmu is an alternative way to provide
> > > device model now. We think that still involving QEmu as a requirement is
> > > somehow strange. So this reserved region is there, to drop the dependence
> > > with QEmu to provide PV driver(PV driver depends on QEmu is still
> > > strange, right?).
> > 
> > So with your patchset you can run an HVM guest with no qemu process at
> > all? What about the other emulated devices which have no PV equivalent?
> > How does the VM boot, does the BIOS have pv INT 13 handler?
> 
> No, not currently... Sorry for confusion. 
> 
> We just think QEmu provided the availability of PV driver seems unelegant, so 
> we want to decouple them. Because what QEmu provided is a PCI IRQ and a MMIO 
> region for grant table. And event channel is available without that PCI IRQ, 
> so we think decouple MMIO from QEmu should be more elegant...

OK, but in that case I think we should have a mechanism for the guest to
query the location of the grant table pages (hypercall, MSR etc) rather
than hardcoding a magic address. It may well end up being hardcoded on
the tools/hypervisor side for now but there is no reason to expose that
to the guest.

I wonder if we could turn things around and have the guest pick some
pages and tell the hypervisor to put the grant table there, removing the
need to reserve any of the physical address space up front. How do
full-PV guests find their grant table, is that a mechanism which could
be re-used here instead of reserving magic regions?

Ian.


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