Anthony Liguori wrote: > cpu_physical_memory_map(). > > But this function has some subtle characteristics. It may return a > bounce buffer if you attempt to map MMIO memory. There is a limited > pool of bounce buffers available so it may return NULL in the event that > it cannot allocate a bounce buffer. > > It may also return a partial result if you're attempting to map a region > that straddles multiple memory slots.
Thanks. I had found this, but was unsure as to wether it was quite what I wanted. (also is it possible to tell when it has (eg.) allocated a bounce buffer?) Basically, I need to get buffer(s) from guest userspace into the hosts address space. The buffers are virtually contiguous but likely physically discontiguous. They are allocated with malloc() and theres nothing I can do about that. The obvious but slow solution would be to copy all the buffers into nice virtio-based scatter/gather buffers and feed them to the host that way, however its not fast enough. Right now I have a little driver I have written that allows a buffer to be mmap()ed by the guest userspace, and this is pushed to the host via virtio s/g io when the guest calls fsync(). This buffer contains the data that must be passed to the host, however this data may often contain pointers to (that is, userspace virtual addresses of) buffers of unknown sizes which the host also needs to access. These buffers are what I need to read from the guests RAM. The buffers will likely remain active across multiple different calls to the host so their pages will need to be available. As the calls always happen when that specific process is active, I'd guess the worst we need to do is generate a page fault to unswap the page(s). Can that be caused by qemu (under kvm)? It seems that cpu_physical_memory_map() deals with physically contiguous areas of guest address space. I need to get a host-side mapping of a *virtually* contiguous (possibly physically discontiguous) set of guest pages. If this can be done, it'd mean direct transfer of data from guest application to host shared library, which would be a major win. -Ian