[Xen-devel] [PATCH v2] SVM: limit GIF=0 region

["Jan Beulich" <JBeulich@xxxxxxxx>]

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

A note regarding the main STI(s) 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.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Add CLI after VMRUN. Adjust description.

--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/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)
 
@@ -87,7 +87,12 @@ __UNLIKELY_END(nsvm_hap)
         pop  %rsi
         pop  %rdi
 
+        CLGI
+        sti
         VMRUN
+        cli
+        STGI
+GLOBAL(svm_stgi_label)
 
         SAVE_ALL
 
@@ -96,13 +101,12 @@ __UNLIKELY_END(nsvm_hap)
         SPEC_CTRL_ENTRY_FROM_HVM    /* Req: b=curr %rsp=regs/cpuinfo, Clob: 
acd */
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
-        STGI
-GLOBAL(svm_stgi_label)
+        sti
         mov  %rsp,%rdi
         call svm_vmexit_handler
         jmp  .Lsvm_do_resume
 
 .Lsvm_process_softirqs:
-        STGI
+        sti
         call do_softirq
         jmp  .Lsvm_do_resume





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