Preserve siginfo on HVC #9 faults

src/core/shim.S

@@ -36,7 +36,10 @@
  *                          shim restores X11 from the saved frame before
  *                          ERET so the EL0 caller never observes this hint.)
  *   #7  MRS trap          (host reads reg from ESR ISS; returns value in x0)
- *   #9  W^X toggle        (x0=FAR, x1=type: 0=exec->RX, 1=write->RW)
+ *   #9  W^X toggle        (x0=FAR, x1=type: 0=exec->RX, 1=write->RW.
+ *                          On return, X8==0 means retry after W^X TLBI;
+ *                          X8==2 means host delivered a signal frame and the
+ *                          shim must drop the saved fault frame.)
  *   #10 BRK from EL0      (SIGTRAP delivery / ptrace-stop; GPRs in frame)
  *   #11 EL0 fault         (SIGSEGV/SIGILL delivery; GPRs in frame.
  *                          On return the host sets X8 to the same TLBI
@@ -1119,6 +1122,8 @@ handle_inst_abort:
 1:  mrs x0, far_el1           /* Faulting address */
     mov x1, #0                /* Type 0 = instruction fault -> flip to RX */
     hvc #9
+    cmp x8, #2
+    b.eq exec_drop_frame
     b tlbi_restore_eret
 
 /* handle_data_abort: EC=0x24: Data abort from lower EL
@@ -1143,6 +1148,8 @@ handle_data_abort:
     mrs x0, far_el1           /* Faulting address */
     mov x1, #1                /* Type 1 = write fault -> flip to RW */
     hvc #9
+    cmp x8, #2
+    b.eq exec_drop_frame
     b tlbi_restore_eret
 
 /* handle_el0_fault: Deliver SIGSEGV/SIGILL for EL0 faults

src/syscall/proc.c

@@ -2065,8 +2065,16 @@ int vcpu_run_loop(hv_vcpu_t vcpu,
                      * so the next syscall epilogue does not re-flush the W^X
                      * page. cpu_tlbi_req is per-vCPU, so this only touches our
                      * own slot -- concurrent vCPUs are unaffected.
+                     *
+                     * The HVC #9 shim now consumes X8 as a post-HVC marker:
+                     * 0 means W^X succeeded and the shim should run the TLBI
+                     * retry epilogue; 2 means signal_deliver_fault installed a
+                     * handler frame and the shim must drop its saved frame.
+                     * Clear X8 here so a guest's pre-fault X8 value cannot be
+                     * misread as the frame-drop marker after a normal toggle.
                      */
                     tlbi_request_clear();
+                    hv_vcpu_set_reg(vcpu, HV_REG_X8, 0);
                     break;
                 }
 

src/syscall/signal.c

@@ -1735,9 +1735,11 @@ static int deliver_signal_locked(hv_vcpu_t vcpu,
     /* If delivery happens while returning from the syscall HVC path, the shim
      * still has the interrupted syscall frame on its EL1 stack. Tell it to drop
      * that frame so the handler PC/SP/LR/args installed above are not
-     * overwritten before ERET. Fault/BRK delivery paths ignore this marker. The
-     * EL0-preemption path resumes straight into the handler at EL0 with no shim
-     * frame to drop, so the marker is neither needed nor consulted.
+     * overwritten before ERET. HVC #9 fault fallback uses the same marker when
+     * it materializes a SIGSEGV frame; HVC #11 and BRK delivery paths do not
+     * consume it. The EL0-preemption path resumes straight into the handler at
+     * EL0 with no shim frame to drop, so the marker is neither needed nor
+     * consulted.
      */
     if (!el0_preempt)
         hv_vcpu_set_reg(vcpu, HV_REG_X8, 2);