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Re: [Xen-devel] [PATCH Remus v2 04/10] tools/libxc: introduce DECLARE_HYPERCALL_BUFFER_USER_POINTER

On 05/11/2015 07:53 PM, Ian Campbell wrote:
On Fri, 2015-05-08 at 17:33 +0800, Yang Hongyang wrote:
Define a user pointer that can access the hypercall buffer data
Useful when you only need to access the hypercall buffer data

Can you expand on this please, I think the context is a hypercall buffer
passed as an argument or as a member of a struct. Please describe why
are not usable here.

There are cases that we only need to use the hypercall buffer data, and do
not use the xc_hypercall_buffer_t struct.
DECLARE_HYPERCALL_BUFFER_ARGUMENT will only define a xc_hypercall_buffer_t,
while DECLARE_HYPERCALL_BUFFER_SHADOW do define a user pointer that can allow
us to access the hypercall buffer data but it also define a
xc_hypercall_buffer_t that we don't use, the compiler will report arg unused

The cases for example:
In send_all_pages(), we only need to use the haypercall buffer data which is a
dirty bitmap, we set the dirty bitmap to all dirty and call send_dirty_pages,
we will not use the xc_hypercall_buffer_t and hypercall to retrive the dirty
In send_some_pages(), we will also only need to use the dirty_bitmap. the
retrive dirty bitmap hypercall are done by the caller.




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