x86/pv: Guest exception handling in FRED mode

Under FRED, entry_from_pv() handles everything.  To start with, implement
exception handling in the same manner as entry_from_xen(), although we can
unconditionally enable interrupts after the async/fatal events.

After entry_from_pv() returns, test_all_events() needs to run to perform
exception and interrupt injection.  Split entry_FRED_R3() into two and
introduce eretu_exit_to_guest() as the latter half, coming unilaterally from
restore_all_guest().

For all of this, there is a slightly complicated relationship with CONFIG_PV.
entry_FRED_R3() must exist irrespective of CONFIG_PV, because it's the
entrypoint registered with hardware.  For simplicity, entry_from_pv() is
always called, but it collapses into fatal_trap() in the !PV case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/traps.c

@@ -2267,9 +2267,85 @@ void asmlinkage check_ist_exit(const struct cpu_user_regs *regs, bool ist_exit)
 
 void asmlinkage entry_from_pv(struct cpu_user_regs *regs)
 {
-    /* Copy fred_ss.vector into entry_vector as IDT delivery would have done. */
+    struct fred_info *fi = cpu_regs_fred_info(regs);
-    regs->entry_vector = regs->fred_ss.vector;
+    uint8_t type = regs->fred_ss.type;
+    uint8_t vec = regs->fred_ss.vector;
 
+    /* Copy fred_ss.vector into entry_vector as IDT delivery would have done. */
+    regs->entry_vector = vec;
+
+    if ( !IS_ENABLED(CONFIG_PV) )
+        goto fatal;
+
+    /*
+     * First, handle the asynchronous or fatal events.  These are either
+     * unrelated to the interrupted context, or may not have valid context
+     * recorded, and all have special rules on how/whether to re-enable IRQs.
+     */
+    if ( regs->fred_ss.nested )
+        goto fatal;
+
+    switch ( type )
+    {
+    case X86_ET_EXT_INTR:
+        return do_IRQ(regs);
+
+    case X86_ET_NMI:
+        return do_nmi(regs);
+
+    case X86_ET_HW_EXC:
+        switch ( vec )
+        {
+        case X86_EXC_DF: return do_double_fault(regs);
+        case X86_EXC_MC: return do_machine_check(regs);
+        }
+        break;
+    }
+
+    /*
+     * With the asynchronous events handled, what remains are the synchronous
+     * ones.  PV guest context always had interrupts enabled.
+     */
+    local_irq_enable();
+
+    switch ( type )
+    {
+    case X86_ET_HW_EXC:
+    case X86_ET_PRIV_SW_EXC:
+    case X86_ET_SW_EXC:
+        switch ( vec )
+        {
+        case X86_EXC_PF:  handle_PF(regs, fi->edata); break;
+        case X86_EXC_GP:  do_general_protection(regs); break;
+        case X86_EXC_UD:  do_invalid_op(regs); break;
+        case X86_EXC_NM:  do_device_not_available(regs); break;
+        case X86_EXC_BP:  do_int3(regs); break;
+        case X86_EXC_DB:  handle_DB(regs, fi->edata); break;
+        case X86_EXC_CP:  do_entry_CP(regs); break;
+
+        case X86_EXC_DE:
+        case X86_EXC_OF:
+        case X86_EXC_BR:
+        case X86_EXC_NP:
+        case X86_EXC_SS:
+        case X86_EXC_MF:
+        case X86_EXC_AC:
+        case X86_EXC_XM:
+            do_trap(regs);
+            break;
+
+        default:
+            goto fatal;
+        }
+        break;
+
+    default:
+        goto fatal;
+    }
+
+    return;
+
+ fatal:
     fatal_trap(regs, false);
 }
 

xen/arch/x86/x86_64/entry-fred.S

@@ -15,9 +15,20 @@ FUNC(entry_FRED_R3, 4096)
         mov     %rsp, %rdi
         call    entry_from_pv
 
+#ifdef CONFIG_PV
+        GET_STACK_END(14)
+        movq    STACK_CPUINFO_FIELD(current_vcpu)(%r14), %rbx
+
+        jmp     test_all_events
+#else
+        BUG     /* Not Reached */
+#endif
+END(entry_FRED_R3)
+
+FUNC(eretu_exit_to_guest)
         POP_GPRS
         eretu
-END(entry_FRED_R3)
+END(eretu_exit_to_guest)
 
         /* The Ring0 entrypoint is at Ring3 + 0x100. */
         .org entry_FRED_R3 + 0x100, 0xcc

xen/arch/x86/x86_64/entry.S

@@ -63,7 +63,7 @@ UNLIKELY_END(syscall_no_callback)
         /* Conditionally clear DF */
         and   %esi, UREGS_eflags(%rsp)
 /* %rbx: struct vcpu */
-test_all_events:
+LABEL(test_all_events, 0)
         ASSERT_NOT_IN_ATOMIC
         cli                             # tests must not race interrupts
 /*test_softirqs:*/
@@ -152,6 +152,8 @@ END(switch_to_kernel)
 FUNC_LOCAL(restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
 
+        ALTERNATIVE "", "jmp eretu_exit_to_guest", X86_FEATURE_XEN_FRED
+
         /* Stash guest SPEC_CTRL value while we can read struct vcpu. */
         mov VCPU_arch_msrs(%rbx), %rdx
         mov VCPUMSR_spec_ctrl_raw(%rdx), %r15d