Confusion about copy_to/from_guest

Confusion about copy_to/from_guest

[Tej]

All,

Small and naive question about the semantics of copy_to/from_guest. In
Kernel space I have created a linked list lets say 10 elements. Now I
have created similar list in hypervisor.

Now passing the list_head from kernel using hypercall. Now how do I
copy the element between kernel and hypervisor.

1. copy_from_guest(hyp_list_head, arg, 1) or
copy_from_guest(hyp_list_head, arg, <num_of_elements_list>)
2. copy_to_guest(arg, hyp_list_head, 1) or copy_to_guest(arg,
hyp_list_head, <num of element in list>)

2nd question:
I did first one, but first element of list was not accessible in
hypervisor space, why?


It looks like I am doing something horribly wrong, so can someone
point out that.

Thanks for any help
-tej

Re: Confusion about copy_to/from_guest

[Keir Fraser]

On 26/04/2010 03:32, "Tej" <bewith.tej@gmail.com> wrote:

> Now passing the list_head from kernel using hypercall. Now how do I
> copy the element between kernel and hypervisor.
> 
> 1. copy_from_guest(hyp_list_head, arg, 1) or
> copy_from_guest(hyp_list_head, arg, <num_of_elements_list>)
> 2. copy_to_guest(arg, hyp_list_head, 1) or copy_to_guest(arg,
> hyp_list_head, <num of element in list>)

The first of these. Copy_{to,from}_guest can copy arrays of structures, but
since this is a linked structure you'd have to do the copy one element at a
time.

> 2nd question:
> I did first one, but first element of list was not accessible in
> hypervisor space, why?

Well, if you are really only copying the list_head, the above should work.

 -- Keir

Re: Confusion about copy_to/from_guest

[Tej]

On Mon, Apr 26, 2010 at 11:03 AM, Keir Fraser <keir.fraser@eu.citrix.com> wrote:
> On 26/04/2010 03:32, "Tej" <bewith.tej@gmail.com> wrote:
>
>> Now passing the list_head from kernel using hypercall. Now how do I
>> copy the element between kernel and hypervisor.
>>
>> 1. copy_from_guest(hyp_list_head, arg, 1) or
>> copy_from_guest(hyp_list_head, arg, <num_of_elements_list>)
>> 2. copy_to_guest(arg, hyp_list_head, 1) or copy_to_guest(arg,
>> hyp_list_head, <num of element in list>)
>
> The first of these. Copy_{to,from}_guest can copy arrays of structures, but
> since this is a linked structure you'd have to do the copy one element at a
> time.

you mean to say, in hypervisor space copy first element using
copy_from_guest(hyp_list_head, arg, 1), extract the next address from
it and then copy again the next element using
"copy_from_guest(hyp_list_head->next, arg->next, 1)" ???



>
>> 2nd question:
>> I did first one, but first element of list was not accessible in
>> hypervisor space, why?
>
> Well, if you are really only copying the list_head, the above should work.
Yes, I have carried out the sample test, by allocating small struct
linked list and then passing between hypervisor and kernel
copy_from_guest(hyp_list_head, arg, 1) is working correctly.

But my actial code is not working. Ok let me put some more effort, if
not I will push the code for review.

Thanks for your help

>
>  -- Keir
>
>
>