On Fri, Jan 15, 2010 at 7:58 PM, Jim Keniston jkeni...@us.ibm.com wrote:
4. Emulation removes the need for the XOL area, but requires pretty much
total knowledge of the instruction set. It's also a performance win for
architectures that can't do #3. I see kvm implemented on 4
architectures (ia64, powerpc, s390, x86). Coincidentally, those are the
architectures to which uprobes (old uprobes, with ubp and xol bundled
in) has already been ported (though Intel hasn't been maintaining their
ia64 port). So it sort of comes down to how objectionable the XOL vma
(or page) really is.
On x86 at least, wouldn't one option to be to run the instruction to
be emulated in CPL ('ring') 2, from a XOL page above the user-kernel
split, not accessible to userspace at CPL 3? Linux hasn't
traditionally used anything other than CPL 0 and CPL 3 (plus CPL 1 on
Xen), but it would seem to avoid many of the problems here - it's
invisible to normal userspace code and so doesn't pollute userspace
memory maps with kernel-private stuff, but since it's running at a
higher CPL than the kernel, we can still protect kernel memory and
protect against privileged instructions.
