SVM: limit GIF=0 region

Use EFLAGS.IF for most ordinary purposes; there's in particular no need
to unduly defer NMI/#MC. Clear GIF only immediately before VMRUN itself.
This has the additional advantage that svm_stgi_label now indeed marks
the only place where GIF gets set.

Note regarding the main STI placement: Quite counterintuitively the
host's EFLAGS.IF continues to have a meaning while the guest runs; see
PM Vol 2 section "Physical (INTR) Interrupt Masking in EFLAGS". Hence we
need to set the flag for the duration of time being in guest context.
However, SPEC_CTRL_ENTRY_FROM_HVM wants to be carried out with EFLAGS.IF
clear.

Note regarding the main STGI placement: It could be moved further up,
but at present SPEC_CTRL_EXIT_TO_HVM is not NMI/#MC-safe.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

xen/arch/x86/hvm/svm/entry.S

@@ -43,7 +43,7 @@ ENTRY(svm_asm_do_resume)
         lea  irq_stat+IRQSTAT_softirq_pending(%rip),%rdx
         xor  %ecx,%ecx
         shl  $IRQSTAT_shift,%eax
-        CLGI
+        cli
         cmp  %ecx,(%rdx,%rax,1)
         jne  .Lsvm_process_softirqs
 
@@ -57,7 +57,7 @@ UNLIKELY_START(ne, nsvm_hap)
          * Someone shot down our nested p2m table; go round again
          * and nsvm_vcpu_switch() will fix it for us.
          */
-        STGI
+        sti
         jmp  .Lsvm_do_resume
 __UNLIKELY_END(nsvm_hap)
 
@@ -87,6 +87,8 @@ __UNLIKELY_END(nsvm_hap)
         pop  %rsi
         pop  %rdi
 
+        CLGI
+        sti
         VMRUN
 
         SAVE_ALL
@@ -103,6 +105,6 @@ GLOBAL(svm_stgi_label)
         jmp  .Lsvm_do_resume
 
 .Lsvm_process_softirqs:
-        STGI
+        sti
         call do_softirq
         jmp  .Lsvm_do_resume