[PATCH] x86/emul: Remove fallback path from SWAPGS

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

In real hardware, accesses to the registers cannot fail.  The error paths are
just an artefact of the hook functions needing to return something.

The best effort unwind is also something that doesn't exist in real hardware,
and complicates following the logic.

Instead, use an ASSERT_UNREACHABLE() with a fallback of injecting #DF.
Hitting this path is an error in Xen.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Tested using LKGS's extention of the test emulator for SWAPGS.
---
 xen/arch/x86/x86_emulate/0f01.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/0f01.c b/xen/arch/x86/x86_emulate/0f01.c
index 6c10979dd650..760f5f865913 100644
--- a/xen/arch/x86/x86_emulate/0f01.c
+++ b/xen/arch/x86/x86_emulate/0f01.c
@@ -192,18 +192,21 @@ int x86emul_0f01(struct x86_emulate_state *s,
         if ( (rc = ops->read_segment(x86_seg_gs, &sreg,
                                      ctxt)) != X86EMUL_OKAY ||
              (rc = ops->read_msr(MSR_SHADOW_GS_BASE, &msr_val,
-                                 ctxt)) != X86EMUL_OKAY ||
-             (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base,
-                                  ctxt, false)) != X86EMUL_OKAY )
+                                 ctxt)) != X86EMUL_OKAY )
             goto done;
-        sreg.base = msr_val;
-        if ( (rc = ops->write_segment(x86_seg_gs, &sreg,
-                                      ctxt)) != X86EMUL_OKAY )
+        if ( (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base,
+                                  ctxt, false)) != X86EMUL_OKAY ||
+             (sreg.base = msr_val,
+              (rc = ops->write_segment(x86_seg_gs, &sreg,
+                                       ctxt)) != X86EMUL_OKAY) )
         {
-            /* Best effort unwind (i.e. no real error checking). */
-            if ( ops->write_msr(MSR_SHADOW_GS_BASE, msr_val,
-                                ctxt, false) == X86EMUL_EXCEPTION )
-                x86_emul_reset_event(ctxt);
+            /*
+             * In real hardware, access to the registers cannot fail.  It is
+             * an error in Xen if the writes fail given that both MSRs have
+             * equivalent checks.
+             */
+            ASSERT_UNREACHABLE();
+            generate_exception(X86_EXC_DF);
             goto done;
         }
         break;
-- 
2.39.5