Hello Julien,
On 03.05.19 20:19, Julien Grall wrote:
TLDR; because xen page-tables are not that often modified after boot. Yet it is
still possible to race.
At the moment, create_xen_entries() can only modify the VA range 0 - 2GB. In
that range, we can modify at runtime the VMAP area. One potential issue is
a vmap issued at the same time.
While the range allocation is protected by a lock (see vm_alloc), the mapping
is not. So it would be possible to end up modifying the page-table at the same.
That could blow up if for instance, the second-level entry is invalid as we
would need to allocate memory (only one can win that race).
I understand the potential race, but still wondering why didn't we see those
issues. Maybe we are too lucky.
In general, it is a saner approach to try to serialize the modifications in the
page-tables. So you can safely read an entry, check it and then update it.
Yet, I think we would stick at these locks for now.
--
Sincerely,
Andrii Anisov.
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