Re: [Xen-devel] Confused about Xen memory management.

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> OK, so I am implementing a prototype where Linux hands an address into
> Xen, which gets decremented later (ie. when Xen is executing a different
> domain).  It seems that map_domain_mem() is what I want, but how do I
> get the physical address of the variable?  virt_to_phys() in Linux
> doesn't seem to do the trick, nor virt_to_phys() in Xen.

In the Linux driver use virt_to_machine(). Then pass that machine
address thru map_domain_mem() in Xen to get a temporary virtual
mapping. If you care about safety (does teh machine address beliong to
the domain?) then you could do a get_page() to ensure the domain
really owns the page.

If you know that Xen will only access the variable in your domain's
context (i.e., whil eusing that domain's page tables) then you might
get away with passing down a virtual address and directly derefercing
that (perhap susing get_user/put_user for safety).

 -- Keir

> ie, pseudo code:
> 
> linux/mydriver.c:
>       static int foo = 1;
> 
>       // virt_to_phys(foo) maybe?
>       HYPERVISOR_mytest(&foo);
> 
> xen/common/test.c:
>       int mytest(unsigned long addr)
>       {
>               // virt_to_phys(addr) maybe?
>               manip_later->addr = addr;
>               return 0;
>       }
> 
>       // Some time later
>       ...
>       int *p = map_domain_mem(manip_later->addr);
>       (*p)++;
>       unmap_domain_mem(p);
> 
> Sorry for the stupid question; I hate VM.
> Rusty.
> -- 
> A bad analogy is like a leaky screwdriver -- Richard Braakman
> 
> 
> 
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