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

[Hollis Blanchard <hollisb@xxxxxxxxxx>]

On Aug 19, 2005, at 9:35 AM, Keir Fraser wrote:
> On 19 Aug 2005, at 14:57, Hollis Blanchard wrote:
>
> > - replace libxc calls to mlock() with register_this_address() (which 
> > could be a privcmd ioctl)
> Yep. I think libxc would request via a privcmd ioctl. The kernel can 
> extend the parameter-passing region, or allocate a subsection of the 
> existing region, and mmap it into user space. It would also return to 
> libxc the range of parameter-passing addresses that have been 
> allocated to it.
> > - libxc ignorantly creates its structures with userspace addresses
> libxc would create structs with parameter-passing addresses.
Does "parameter-passing addresses" mean offsets inside the parameter 
passing space?
I think pseudocode is going to be more effective than English here. 
Let's take DOM0_PERFCCONTROL as an example:
main() {
    xc_perfc_desc_t *desc = malloc();
    mlock(desc); // <------------- [1]
    xc_perfc_control(desc);
}

xc_perfc_control(xc_perfc_desc_t *desc) {
    dom0_op_t dop;

    dop.cmd = DOM0_PERFCCONTROL;
    dop.u.perfccontrol.desc = desc; // <------------ [2]
    do_dom0_op(&dop);
}

Even if you replace malloc/mlock at [1] with a call that maps 
"parameter passing" space into this process, what address will you put 
in the struct at [2]? That would have to be an offset within the 
parameter passing space, right?
--
Hollis Blanchard
IBM Linux Technology Center


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