Re: [Xen-devel] passing hypercall parameters by pointer

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Aug 19, 2005, at 7:17 AM, Keir Fraser wrote:
> On 19 Aug 2005, at 12:52, Jimi Xenidis wrote:
>
> > IP> Since passing by VA is so convenient on the architectures that
> >  IP> support it we may not want to do anything different on these
> >  IP> anyhow.
> >
> > I agree, why create a new mapping when a usable one exists.
> >
> > At least for common kernel code, we will need to wrap such VAs in a
> > macro so that the "psuedo-physical" is passed in for PPC. I assume
> > this is reasonable?
> This is all potentially fixable before 3.0 final. Paravirt x86 can 
> continue to use guest virtual addresses. The idea would be that the 
> registration scheme would essentially create a parameter-passing 
> 'address space' into which you hook pages of memory. On x86 we would 
> map the address space onto regions of kernel va space. On other arches 
> we would map the address space onto physical addresses that get mapped 
> into Xen's va space. get_user/put_user/copy_from_user/copy_to_user 
> will take guest addresses that point into this parameter-passing 
> address space.
Could you flesh this out a little more? I *think* what you're saying is 
this (on PowerPC):
- at boot, the kernel notifies Xen of a parameter page
- replace libxc calls to mlock() with register_this_address() (which 
could be a privcmd ioctl)
- register_this_address() stuffs the userspace pointer and 
corresponding pseudo-physical pointer into a table in the parameter 
page
- libxc ignorantly creates its structures with userspace addresses
- once the hypercall arrives in Xen, copy_from_user() is passed the 
userspace address
- copy_from_user() consults the table in the parameter page to 
translate userspace -> pseudo-physical, then translates pseudo-physical 
-> machine
Is that right?

--
Hollis Blanchard
IBM Linux Technology Center


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