Re: [RFC PATCH 6/8] kvm: gmem: Temporarily restore direct map entries when needed

[Paolo Bonzini]

On 7/9/24 15:20, Patrick Roy wrote:
> If KVM_GMEM_NO_DIRECT_MAP is set, and KVM tries to internally access
> guest-private memory inside kvm_{read,write}_guest, or via a
> gfn_to_pfn_cache, temporarily restore the direct map entry.
>
> To avoid race conditions between two threads restoring or zapping direct
> map entries for the same page and potentially interfering with each
> other (e.g. unfortune interweavings of map->read->unmap in the form of
> map(A)->map(B)->read(A)->unmap(A)->read(B) [BOOM]), the following
> invariant is upheld in this patch:
>
> - Only a single gfn_to_pfn_cache can exist for any given pfn, and

I think this is not ensured.  You can however use 
set_page_private()/page_private() to count the number of references.

Paolo

> - All non-gfn_to_pfn_cache code paths that temporarily restore direct
>    map entries complete the entire map->access->unmap critical section
> while holding the folio lock.