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[Xen-devel] LTTng-Xen Buffer shared between the hypervisor and a dom0 process


My LTTng-for-Xen implementation is almost ready, except for one detail :

I would like to free a memory region shared between the hypervisor and
lttd-xen (a dom0 process). Ideally, it should be freed when the process
unmaps the memory, is killed or exits.

In my current implementation,

The hypervisor allocates the buffer upon trace creation hypercall :

        /* Share pages so that lttd-xen can map them. */
        for ( i = 0; i < nr_pages; i++ )
                        virt_to_page(rawbuf) + i, XENSHARE_writable);

the process maps the memory with :

            buffer = xc_map_foreign_range(xc_handle, DOMID_XEN,
                          pair->num_cpus * pair->n_subbufs * pair->subbuf_size,
                          PROT_READ, pair->mfn);

Then, I would like to release some kind of reference count of this
mapping from the hypervisor. I do the following which results in page
faults (probably because it tries to free memory still accessed by
lttd-xen) :

                get_order_from_bytes(chan->alloc_size * num_possible_cpus()));

And then, when we are sure that no more data can be written in the
buffer, lttd-xen is ready to exit. It unmaps the buffer just before exit :

            err_ret = munmap(pair->mmap, pair->subbuf_size * pair->n_subbufs);

Do you know any proper way to achieve what I am looking for ?



Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

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