Right now we have two IRET instructions that can fault for guest reasons, and
the pre exception table gives handle_exception as the fixup for both.

Instead, we can have compat_restore_all_guest() use restore_all_guest()'s IRET
which gives us just a single position to handle specially.

In exception_with_ints_disabled(), remove search_pre_exception_table() and use
a simpler check.  Explain how the recovery works, because this isn't the first
time I've had to reverse engineer it for my own understanding.

The reference to iret_to_guest highlights that any checking here is specific
to CONFIG_PV, so exclude it in !PV builds.

Later in exception_with_ints_disabled(), it suffices to load %ecx rather than
%rcx, and remove a stray semi-colon from the rep movsq.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>

v2:
 * New
---
 xen/arch/x86/x86_64/compat/entry.S |  3 +--
 xen/arch/x86/x86_64/entry.S        | 31 ++++++++++++++++++++++--------
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index d7b381ea546d..39925d80a677 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -167,8 +167,7 @@ FUNC(compat_restore_all_guest)
             scf=STK_REL(CPUINFO_scf,      CPUINFO_rip), \
             sel=STK_REL(CPUINFO_verw_sel, CPUINFO_rip)
 
-.Lft0:  iretq
-        _ASM_PRE_EXTABLE(.Lft0, handle_exception)
+        jmp     iret_to_guest
 END(compat_restore_all_guest)
 
 /* Callers can cope with both %rax and %rcx being clobbered. */
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index c02245ac064c..01b431793b7b 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -241,8 +241,9 @@ iret_exit_to_guest:
         SPEC_CTRL_COND_VERW     /* Req: %rsp=eframe                    Clob: efl */
 
         addq  $8,%rsp
-.Lft0:  iretq
-        _ASM_PRE_EXTABLE(.Lft0, handle_exception)
+
+LABEL(iret_to_guest, 0)
+        iretq
 END(restore_all_guest)
 
 /*
@@ -920,10 +921,23 @@ handle_exception_saved:
 exception_with_ints_disabled:
         testb $3,UREGS_cs(%rsp)         # interrupts disabled outside Xen?
         jnz   FATAL_exception_with_ints_disabled
-        movq  %rsp,%rdi
-        call  search_pre_exception_table
-        testq %rax,%rax                 # no fixup code for faulting EIP?
-        jz    .Ldispatch_exceptions
+
+#ifndef CONFIG_PV
+        /* No PV?  No IRETs-to-guest to worry about. */
+        jmp .Ldispatch_exceptions
+#else
+        /* Check to see if the exception was on the IRET to guest context. */
+        lea   iret_to_guest(%rip), %rax
+        cmp   %rax, UREGS_rip(%rsp)
+        jne   .Ldispatch_exceptions
+
+        /*
+         * Recovery is at handle_exception.  It may be necessary to make space
+         * on the interrupted stack for ec/ev, after which the current ec/ev
+         * is copied to make it appear as if this exception occurred in guest
+         * context.
+         */
+        lea   handle_exception(%rip), %rax
         movq  %rax,UREGS_rip(%rsp)      # fixup regular stack
 
 #ifdef CONFIG_XEN_SHSTK
@@ -940,13 +954,14 @@ exception_with_ints_disabled:
         movq  %rsp,%rsi
         subq  $8,%rsp
         movq  %rsp,%rdi
-        movq  $UREGS_kernel_sizeof/8,%rcx
-        rep;  movsq                     # make room for ec/ev
+        mov   $UREGS_kernel_sizeof/8, %ecx
+        rep movsq                       # make room for ec/ev
 1:      movq  UREGS_error_code(%rsp),%rax # ec/ev
         movq  %rax,UREGS_kernel_sizeof(%rsp)
         mov   %r15, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         mov   %r13b, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
         jmp   restore_all_xen           # return to fixup code
+#endif /* !CONFIG_PV */
 
 /* No special register assumptions. */
 FATAL_exception_with_ints_disabled:
-- 
2.39.5