Re: question regarding foreign memory mapping in xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 27/05/13 14:06, Sengul Thomas wrote:
> Hello xen developers :)
>
> I'm reading source codes of xen and got a question in tool-stack part
> and got series of questions:
>
> In file tools/libxc/xc_linux_osdep.c, function linux_privcmd_map_foreign_bulk,
> I notice that mmap and ioctl IOCTL_PRIVCMD_MMAPBATCH_V2, are used
> for mapping a foreign memory into dom0.
>
> Is it because mmap is for mapping from /proc/xen/privcmd to dom0 memory and
> ioctl is for mapping from foreign memory (probably domU) into /proc/xen/privcmd?
>
> Also, I am trying to understand how ioctl IOCTL_PRIVCMD_MMAPBATCH_V2 works,
> and it is quite hard.
> In function privcmd_ioctl_mmap_batch, it allocates pages in dom0 side,
> and I don't understand why do we need this allocation.
> If we map a page from foreign domain, just changing grant table is not enough?
> Please enlighten me.

A foreign mapping is unrelated to grant tables.

Grant tables are used for a domain to negotiate to allow other domains
access to parts of its memory.

Foreign mappings are for the toolstack needing to play with a domains
memory without its knowledge.  It is the basis of domain create/migrate etc.

~Andrew

>
> Also, I see that in function mmap_batch_fn (i.e., called with the
> newly allocated pages),
> xen_remap_domain_mfn_range function is called.
> Could you tell me what does this function (xen_remap_domain_mfn_range)
> do briefly.
> It would help me understand xen more!
>
> Best regards,
> Thomas
>
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