On 01.08.24 00:21, Elliot Berman wrote:
I wanted to revive this thread based on the mm alignment discussion for guest_memfd. Gunyah's guest_memfd allocates memory via filemap_alloc_folio, identical to KVM's guest_memfd. There's a possiblity of a stage-2 fault when memory is donated to guest VM and Linux incidentally tries to access the donated memory with an unaligned access. This access will cause kernel to panic as it expects to be able to access all memory which has been mapped in stage 1. We don't want to disallow unaligned access simply because Gunyah drivers are enabled. There are two options I see to prevent the stage-2 fault from crashing the kernel: we can fix up the stage-2 fault or ensure that Linux has a S1 table consistent with S2. To do the latter, the obvious solution seemed to be using the set_direct_map functions, but you and Christoph have valid concerns about exporting this to modules since it's a low-level API. One way to avoid exporting the symbols is to make Gunyah a built-in, but I'd like to find a better solution. One way I can think of is to create a "guest_memfd library" that both KVM and Gunyah can use. It abstracts the common bits between the 2 into a built-in module and can be the one to call the set_direct_map functions. I also think the abstraction will also help keep KVM guest_memfd cleaner once we start supporting huge folios (and splitting them). Do KVM and mm folks also see value to using a library-fied guest_memfd?
Without knowing about any details, this sounds like the right approach to me!
-- Cheers, David / dhildenb
