Hello, In Xen x86, there is the shadow paging that could be used in some cases (notably with HVM), and is used when HAP is not available. It may also be used in very specific cases for PV guests.
With the vast majority of hardware supporting HAP (through EPT or NPT), it's rarely used in practice for HVM, with most downstream projects (XenServer, XCP-ng, QubesOS) are compiling-out its support by default. It also performs very significantly worse than HAP. Therefore, it's not very used in production, and unsurprisingly isn't actively being worked on (most projects being around HAP). One of the issues of Shadow Paging is that it relies on a heavy set of optimizations, with some of them being effectively obselete (optimisations based on heuristics designed for more than 20 years old kernels) or eventually problematic (e.g with modern CPU mitigations e.g L1TF mitigations). Most of these optimizations are hard to reason regarding reliability and security and difficult to debug. Some of these optimizations causes very subtle issues with TLB refactoring for ASID management rework [1]. Moreover, it's fair to say performance is no longer the priority for shadow paging. My proposal would be to : - significantly reduce the complexity of shadow paging by dropping most of the complex optimizations - consider Shadow Paging as Deprecated > Functional completeness: Yes > Functional stability: Quirky > Interface stability: No (as in, may disappear the next release) > Security supported: Yes Teddy [1] https://lore.kernel.org/xen-devel/9cdb3e67abd01390bcc4cd103ca539d6bf7adbc0.1747312394.git.teddy.as...@vates.tech/ | Vates XCP-ng & Xen Orchestra - Vates solutions web: https://vates.tech
