Rusty Russell wrote: > Hi all, > > The next obvious step for paravirt_ops seems to me to be higher-level > mmu operations: from reading the VMI patches it seems to do flushing, > whereas Xen opts for batching. > > In the spirit of ops structures, this would be done by putting > higher-level operations into the ops structure, and batching done by the > op itself (perhaps with a default implementation for those too lazy to > implement it). At this point we run into the brick wall of my > ignorance: Chris, Zach, anyone, are there any obvious interfaces from a > Linux POV? > I would say that attempting to move MMU operations into an ops structure is still premature. There are substantial differences in the set of operations that would be useful to a shadow mode Xen guest, a shadow mode VMI guest, and a direct mode Xen guest. Writable pagetables is I believe still an optional interface for UP guests in Xen. So there are a lot of combinations, and picking the set of ops we need to hook needs a lot of discussion. I think for writable page tables, Xen needs only a set_pte function, but it does need two higher level abstractions - to iterate, change protection and register pages prior to switching to an address space, and to move off the current address space onto swapper_pgdir before destroying an address space. A VMI guest needs only low level page table operations. A shadow mode Xen guest may benefit from some mixed set of operations - of which I confess not to know the best approach. I don't know how important shadow mode is to Xen. So now we are mixing low-level and high-level operations, and we need a good plan laid out. Lets hope we can promote some more discussions along these lines - finding the balance between flexibility, performance, and intrusiveness in the MMU interface is a big problem. I'm working on a proposal along these lines for a well defined set of hooks, but meanwhile I will take a look at these patches and would encourage other proposals as well. Zach
