> If the technical documentation at > http://www.usenix.org/publications/library/proceedings/usenix05/tech/freeni > x/full_papers/bellard/bellard_html/index.html is still valid (I think it > is), Qemu has two modes of handling access to guest memory - system > emulation, in which an entire guest address space is mapped on the host, > and emulated MMU.
No. qemu-fast (using the host address space) was removed long ago. There are a few stray remnants, but nothing useful. We always use an emulated MMU. > I was wondering whether something in-between would also > be feasible. That is, chunks of guest address space (say 4MB chunks for > the sake of the argument) are mmapped into the address space of the Qemu > process on the host, and when an access to guest memory is made, there is > an initial check to see whether it is in the same chunk as the last one, > in which case all the MMU emulation bits could be saved. I could imagine > Qemu keeping a current/most recent chunk for each register which can be > used for relative addressing, plus one for non-register-relative accesses. > It seems to me that this could potentially speed up memory access quite a > bit, and as a bonus even make it easy to support x86 segmentation (as part > of the bounds check for whether a memory access is in a chunk). This is effectively shadow paging implemented in userspace via mmap. It's very hard to make it work in a sane way, and even harder to make it go fast. TLB handling is already a significant bottleneck for many tasks, adding a mmap call is likely to make this orders of magnitude worse. Most guests use virtual memory extensively, so the virtual->physical mappings tend to be extremely fragmented. If you really want to do shadow paging for cross environments, you probably need to move it into kernel space. Either as a host kernel module, or as a bare-metal kernel/application that runs inside KVM. Even then you have to use various tricks to partition off a section of the host address space for use by qemu. It's not impossible, but it is a significant undertaking with somewhat unclear benefits. Paul