Martin Schwidefsky wrote: > On Sun, 2008-03-23 at 12:15 +0200, Avi Kivity wrote: > >>>> Can you convert the page tables at a later time without doing a >>>> wholesale replacement of the mm? It should be a bit easier to keep >>>> people off the pagetables than keep their grubby mitts off the mm >>>> itself. >>>> >>>> >>> Yes, as far as I can see you're right. And whatever we do in arch code, >>> after all it's just a work around to avoid a new clone flag. >>> If something like clone() with CLONE_KVM would be useful for more >>> architectures than just s390 then maybe we should try to get a flag. >>> >>> Oh... there are just two unused clone flag bits left. Looks like the >>> namespace changes ate up a lot of them lately. >>> >>> Well, we could still play dirty tricks like setting a bit in current >>> via whatever mechanism which indicates child-wants-extended-page-tables >>> and then just fork and be happy. >>> >>> >> How about taking mmap_sem for write and converting all page tables >> in-place? I'd rather avoid the need to fork() when creating a VM. >> > > That was my initial approach as well. If all the page table allocations > can be fullfilled the code is not too complicated. To handle allocation > failures gets tricky. At this point I realized that dup_mmap already > does what we want to do. It walks all the page tables, allocates new > page tables and copies the ptes. In principle I would reinvent the wheel > if we can not use dup_mmap
Well, dup_mm() can't work (and now that I think about it, for more reasons -- what if the process has threads?). I don't think conversion is too bad. You'd need a four-level loop to allocate and convert, and another loop to deallocate in case of error. If, as I don't doubt, s390 hardware can modify the ptes, you'd need cmpxchg to read and clear a pte in one operation. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
