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The latter cannot take place without the former. > > > > > > I assume cache maintenance only requires _a_ mapping to the physical memory. > > > With guest_memfd, KVM has the pfn (which happens to always be struct page memory > > > today), and so can establish a VA=>PA mapping as needed. > > > > This is why we are forcing FWB in this work, because we don't have a > > VA mapping and KVM doesn't have the code to create one on demand. > > I don't follow. As it exists today, guest_memfd doesn't touch the direct map, > i.e. there's already a kernel mapping, KVM doesn't need to create one. This is not about guest_memfd.. When ARM KVM copies a PTE from a VMA's PTE into the S2 it may find that the physical address does not have a struct page, in which case it assumes the worst that it is not in the kmap and cannot be cache flushed. Jason