x86/64: paravirt 32-on-64 call gate support

As we realized while trying out NetWare's ring 3 support, call gates
didn't work for 32-bit guests on 64-bit hypervisor. Since x86-64
doesn't know 16- or 32-bit call gates, the only option was to emulate
them. The code here was developed against 3.0.4, so hasn't been
checked for potential integration possibilities with the much improved
emulator; nevertheless I want to supply this patch.

As was realized in the course of creating this patch, 64-bit gates
don't work either, and will also need to be emulated if any
environment intends to use them. The patch changes behavior here in
that rather silently permitting the use of 64-bit gates (with possibly
difficult to understand exceptions happening on the first instruction
of the call/jump target) the call/jump itself will now fault, with the
error code indicating the gate that was attempted to be used. I intend
to complete the emulation to also cover 64-bit gates, but there is one
issue that first needs to be addressed: Whether a gate transitions
from user to kernel mode doesn't depend on the gate, but rather on the
descriptor referenced by the selector held in the gate. As the two can
change independently, this decision can be made only at the point of
use of the gate, and consequently descriptors for kernel code segments
must become distinguishable from user ones, which they currently
aren't as they both get their DPL forced to 3. An initial thought here
is to possibly leverage the otherwise meaningless conforming bit
(i.e. forcing it on for all user code segments, and off for kernel
ones, where then the distinction can be made at the point the
descriptor gets verified/fixed up based of the kernel supplied DPL
[wouldn't work for old guests when setting the DPL to 3 was still
required to be done by the guest]).

The patch also changes behavior of check_descriptor() in that no
modification is done to the descriptor anymore unless all verification
steps passed, and in that the selector RPL of selectors in call gates
no longer gets fixed up (a comment elsewhere in the code correctly
states that the RPL field here isn't used for anything by the
processor); really, this field is now used on 64-bits to store the
original DPL of the gate, because the architectural one now gets
forced to zero.

Signed-off-by: Jan Beulich <jbeulich@novell.com>

0 files changed, 0 insertions, 0 deletions