[librm] Speed up protected-to-real mode transition under KVM

On an Intel CPU supporting VMX, KVM will emulate instructions while
the CPU state remains "invalid".  In real mode, the CPU state is
defined to be "invalid" if any segment register has a base which is
not equal to (sreg<<4) or a limit which is not equal to 64kB.

We don't actually use the base stored in the REAL_DS descriptor for
any significant purpose.  Change the base stored in this descriptor to
be equal to (REAL_DS<<4).  A segment register loaded with REAL_DS is
then automatically valid in both real and protected modes.  This
allows KVM to stop emulating instructions much sooner.

The only use of REAL_DS for memory accesses currently occurs in the
indirect ljmp within prot_to_real.  Change this to a direct ljmp,
storing rm_cs in .text16 as part of the ljmp instruction.  This
removes the only memory access via REAL_DS (thereby allowing for the
above descriptor base address hack), and also simplifies the ljmp
instruction (which will still have to be emulated).

Load the real-mode interrupt descriptor table register before
switching to real mode, since this avoids triggering an EXCEPTION_NMI
and corresponding VM exit.

This reduces the time taken by prot_to_real under KVM by around 65%.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/arch/i386/include/librm.h b/src/arch/i386/include/librm.h
index 4a4e61aa..c8ba72b5 100644
--- a/src/arch/i386/include/librm.h
+++ b/src/arch/i386/include/librm.h
@@ -165,8 +165,8 @@ extern char *text16;
 /* Variables in librm.S, present in the normal data segment */
 extern uint16_t rm_sp;
 extern uint16_t rm_ss;
-extern uint16_t __data16 ( rm_cs );
-#define rm_cs __use_data16 ( rm_cs )
+extern uint16_t __text16 ( rm_cs );
+#define rm_cs __use_text16 ( rm_cs )
 extern uint16_t __text16 ( rm_ds );
 #define rm_ds __use_text16 ( rm_ds )