hi,

what's wrong with just mapping the page twice in domain0?

-Chuck


On Sun, Sep 11, 2005 at 06:42:07PM +0200, Manuel Bouyer wrote:
> Hi,
> this is a question for UVM gurus :)
> I'm trying to implement 0-copy in xen's network backend, and have to solve
> a VM issue.
> 
> Background: when a guest transmits a packet via the hypervisor to
> the domain0's network backend, it gives domain0 a descriptor containing
> a physical address and a size. domain0 has to map this in its own VM space
> to be able to read it. Note that the page passed to domain0 is not part
> of domain0's physical address space, and so not managed by the pmap
> (it's mapped directly by calls to the hypervisor, using a virtual address
> range reserved via uvm_km_alloc(), see xen/i386/xen_shm_machdep.c). This is
> used by both the network and disk back-ends.
> Maybe this could change and let UVM manage it, but I don't know how, or even
> if it would be a good thing.
> 
> Now the problem I have to solve: the network backend can receive 2
> requests which share a page. Right now it's not a problem because for
> each request, the page is mapped, the packet copied and the page unmapped
> in an atomic manner. My goal is to avoid copy if possible, by attaching
> the data to a mbuf using MEXTADD(). This cause problem, because we may have
> to handle a request which is in a page which is already mapped. So I have to:
> - find if a given physical address is already mapped at some VA.
> - use a reference count for these VAs, in order to know when it can be
>   unmapped.
> 
> Is there some mechanism in UVM that could fix both of these problems,
> and would be useable without too much work ? 
> 
> -- 
> Manuel Bouyer <bouyer@antioche.eu.org>
>      NetBSD: 26 ans d'experience feront toujours la difference
> --